I'm trying to think through some gearing options for my 1650lb Exocet.
I'm currently N/A on a VVT engine with 145K miles, a 5 speed and either a 4.30 or 4.10 diff (I'm thinking 4.30 diff because its based on a 02 NB2 which I believe should be 4.30 but the guy who sold me his almost completed project said he thought it was a 4.10)
Needless to say, it gets going quick due to the light weight.

I also have a 91 that I upgraded to a 1.8 Torsen years ago when it was N/A. I had found a 4.10 but everyone said the car would feel really slow so I got an open 4.30 and swapped internals and put the 4.30 Torsen in the 91. Gearing felt great on the track with the N/A power, etc. A few years later I installed a GReddy turbo kit and jumped the power from 105hp to 175hp and all the sudden I felt like I was having to shift a lot more to keep it in the power band. It felt like with the aluminum flywheel and boost I'd just rip right through the rev range and have to shift at less than ideal and drop back out of the power.

I'm getting ready to turbo the Exocet with an EFR kit starting off with the current 145k motor and then eventually with a rebuild and forged rod bottom end.
Initial power goals will be to keeping with a stock engine (220whp/210wtq-ish)
Second engine goals in the 275-300whp range to stay out of the "now you have to start upgrading every piece of your drive train" situation

I've got a 6speed and want to choose a diff that will compliment a light weight car. I mostly use it for track days with the occasional back road blast or taking the long way to work on a nice summer day.
I know the 6speed is not going to be as crisp and easy to shift as the 5speed so it makes sense to me to go with longer gearing as long as the car doesn't start to feel slow.

The 3.9 ratio diff is the stock pairing in the US for a 6speed but would I be better with a 3.6 (I have an option to buy a cheap Fuji and rebuild it with a Torsen)?? Is there going to be much advantage or disadvantage to going with a 3.6 for the track? My local track is Laguna Seca and I often go to Thunder Hill. Now that I have a trailer I'm planning to start going down south to check out some of the tracks down there.

I'm fully comfortable rebuilding a diff with a Torsen unit into an existing pumpkin but don't want to deal with swapping in a different pinion shaft.

 

You won't regret going to a 3.63 rear ratio. You'll have A LOT of torque for that light car and the 3.63 gears will help you take advantage of it. Torsen is a good choice, but also consider a clutch-type LSD as well, as long as you're going to the trouble.

 

I had a 5 speed and 3.91 in my exocet when it was 250whp with the turbo nb motor. Perfect combo and was very happy with it. Not sure how the six speed ratios compare to the 5 speed though.

 

https://www.flyinmiata.com/NB/tech/drivetrain/gearing

Flyin' Miata gearing calculator

 

I'll just leave this here for total gear ratio comparison:

 

Play away: Gearing Calculator - Graf Garage

 

Best Miata gear calculator I've ever seen. Have a cat.

 

Either 5 speed and 3.9 or 6 speed and 3.3. Pretty much the same ratios...
Boileralum has a 3.3 torsen for sale.

NB Torsen LSD w MFactory 3.3 R&P - Miata Turbo Forum - Boost cars, acquire cats.

It's priced right and it was in an Exocet as well.
I'd buy it but my project is a ways off for this.

The 5 speed might live a bit longer in a super light car like an Exocet...
5 speed and 3.9 is the 'inexpensive' route

 

That is exactly what I had and I beat the hell out of that 5-speed and never had any issues with 250whp.