Ok, so I was researching the T5 option and ran across this person who designed a T5 kit to bolt onto a miata.
Mazda Miata Transmission High Preformance Kit for T 5 Z Ford to Miata Motor | eBay
I traded a couple of emails with him and it seems he has been running these on 3 different Miatas for well over a year. Here is some of the email: (Please pardon the grammar errors)
Well I used the t5 because of the 3 inch exhaust sys. one car is the green monster top dog xp car running a 1.8 built by us at Kowalski motor sport running a t 25 60 R turbo and a 8.8 ford IRS diff with the first o s giken super locker every made for the 8.8 she makes 348 bhp 348 tork at the tire on a tire dyno not an axle dyno which reads about 15 % more. local Scca champ car the second is the blue car street car with a fm Stroker motor runs a bit less on power than the top dog car was at 380 but just could not get it to stick even with the 12 inch wide Bogarde on the back with race tires just turn them to hard the third car it my gunners car t-5 8.8 1.6 turbo makeing over 400 at the tire total street car nasty one it is. I did the t5 and 8.8 because every two weeks I blew up a trans or a diff cv axles everything just got tired or fixing **** and since I own a machine shop I just made stuff just for the car to live and in over two years have not broke anything. My car and Goodwin Racing are the two fastest miAtas on the west coast Brian Goodwin a better driver then myself reason I have only been Scca Raceing for four years he twenty five at least well in sandiego last year I did take first he took second for the year as far as build threads I am sixty years old and just not into that kind of stuff I rather be building stuff than shooting my mouth off plus 80% of the **** is ****. I my self have been building race cars since I was 15 well at 15 I was the clean up boy but I was taught by some of the best builders I the world just like my 1.8 in my car it makes full power at 2400 rpm no lag it is so good garret came to see my to find out what I had done to make a no lag turbo car just because of other people that have driven it told garret we want what Stan has, a no lag turbo car that makes big hp, plus 348 bhp is only at 17 pounds of boost we did run it one year at 27 and everything lives but trust me it has a ton of head work which I do know what makes a head work but there is no way any Miata owner will pay 7000.00 for a head because it does take about 120 hours to build one or so and there is only three of them out there as of right now two turbo heads one na head but I don't do this kind of work for a liveing I am a mold maker I just am selling this kit and the kit in my car because next year I am going to go to a automatic trans built to the hilt.
Stan is the owner of Kowalski Machine in Riverside CA. I haven't broken my 6spd (yet), but this like a good option worth further exploring. The gearing is very close to the 5spd we have now.
More possibly usefull-(less) information about the transmission specs.(I couldn't get the chart to copy over here, sorry)
Tremec Transmission (TTC) Specs
And some comments from one of the Mustang forums.
An updated (Reverse Gear TSB installed) T-45 with the Carbon-Fiber synchro rings and the Carbon-Fiber 1-2 rings from a T-56 is a great transmission.
It is a single-rail design like the T-5 and shifts great. They need to be beefed up with the billet 3-4 fork and brass fork pads and a 26 spline input to be top-notch. On the bad side, parts are scarce as hen's teeth. There are no gear ratio options available even for fifth gear. You have what you have. These gears do strip out if you have a lot of torque.
The early T-45's say 96-98 often don't have carbon-fiber synchro rings. Those rings don't hold up and they will decay quickly if one chooses to use the motor oil mentioned above. Using motor oil is also a no-no for a T-5 for the same reason unless it is an 83/84 with the all brass synchro rings.
The stock ones (say 03 COBRA) are not much stronger than the T-45.
It is also a single rail design for smooth shifting. The stock ones can strip out and only have a 27 spline output shaft. The Viper spec T-56 and the Magnum spec are very beefy and might be the prize winner here...
The T-5, T-45 and T-56 were all Borg-Warner designs before Tremec took them over.
This is an old design by Tremec. It is rugged as all getout, but the non-TKO gears are weak and can strip out fairly easily. I don't consider a stock 3550 to be much stronger than a T5Z, but it will take a lot of harsh shifting abuse better than a T-5. Once it has the upgraded gears, 26 spline input and 31 spline output upgrades it is a really strong box and takes a lot of abuse. The synchro rings are brass. It has an awkward 3-rail shifting system that seems to inherently like to jam and miss third gear. Yes, someone made some custom carbon-fiber lined synchro rings for this series, but I'm not sold on them just yet. The TKO-600 is really strong, but they don't shift nicely...
This one is an all Tremec design. It is a 3-rail system, but people don't report having the issues jamming up and missing third like with the TKO. The gears are REALLY strong and beefy. Too much power will WRING OFF the input shaft, but installing a 9310 alloy 26 spline input takes care of that. The early years 01-04 seemed to have trouble with the 3-4-5 & Reverse synchro linings not holding up. The newer rings made after 2004 seem to hold up well though! Many of these had premature failure of the second gear synchros, but that gear and synchro have been redesigned with the "update" kit and that problem is gone. The aluminum forks on these like to break. We are about to see billet forks for the 01-04 hit the market to take care of this!
There are positive shift stops built into this unit so an aftermarket shifter should not have the stops set. All in all, with the upgrades, this one shows real promise, but it is only a 5-speed. There are no gear ratio options other than for fifth gear and then it is still pretty limited in choices there.
This is a 6-speed version of the TR3650 and is currently Tremec's flagship transmission. The Magnum version of it has replaced the T-56 Magnum and is highly regarded, but I don't have much experience inside them yet...
Anyway, I hope you guys find this info helpful.