As was linked to earlier in this thread, there is an installation thread over at Miata.net. I added my own experience and tips to that thread.
Here is my review:
Installed these into a '92 which already had the Boss Frog dual hoop roll bar and very recently had the Boss Frog rails installed. I've also got a (probably worthless) Raceland front shock tower brace. Car rides on stock springs, GR2's and Racing Beat tubular front and adjustable rear sway bar, with FCM 36mm bumps.
Been looking for something to "quiet" down the cowl shudder and general reverberation that seems to keep the front of the car in constant motion.
I didn't notice much change with the roll bar. Perhaps, because the real issue is the front...not the rear. The frame rails seems to stiffen the body but still the front did it's little jig. Can't say I noticed any effect from the cheap shock tower brace (but it looks pretty!)
With my suspension setup, full camber dialed in the front and 1/2 degree less in the rear, the car favored understeer (on the softer rear sway bar setting.) I was planning on moving to the stiffer rear sway bar setting. Running on 14" 195 Falken 512's at 38psi. Also run 225 RS3's, but haven't had enough seat time with them to comment.
The installation is covered under the other thread. Suffice it to say, while there is nothing really complicated about the install, it isn't trivial. It takes a bit of time to free up and remove the fenders...especially if they've never come off the car before, or you shear a body panel bolt (or strip the head like I did.)
I'll cut to the chase and just say the effect is quite impressive. The front end of the car is noticeably stiffer. How much stiffer you might ask. Well, for one thing the added stiffness of the front suspension mounting points has caused my car to go from an understeer condition to neutral/oversteer. There is that much difference in the front end. Steering is much more immediate.
Before I often looked at the front fender and it was a blur of motion....constantly vibrating. Now it appears clear and solid...like an extension of the rest of the car.
Interestingly, front impacts are actually sharper...because the front suspension is stiffer....but they don't reverberate. The cowl shudder is greatly reduced. I need to look at longer bumps with this suspension. Probably hitting them more now because the body is less active while the suspension is more active.
Another indication of the efficacy of the added stiffness is in my CD player, which never was impressive in it's ability to handle impacts...but now skips all the time. The road comes right through to it.
If you are looking for something that will soften your ride, this is not it...quite the opposite.
On the other hand, if you are looking for something that will make your vehicle more responsive...tighten the steering and reduce the shake and shudder, this should be high on your list. This has made more difference to chassis stiffness and handling than any other change I have done. These should be at the top of anyone's list of chassis improvements.
1. Reduces shake and shudder of the front end.
2. Improves steering response.
3. Impacts are quickly damped out.
1. Stiffens the front end which can make impacts more harsh and increase suspension travel.
2. The change in front stiffness may change the cars propensity to understeer or oversteer, requiring spring or sway bar adjustment.
Because I already had a roll bar, frame rails and strut tower brace installed I do not know the effect of just the Frog Arms. Perhaps they are much more effective when coupled with frame rails.