Finding a 84/85 GSL rear end would yield you thicker axles. Not sure if it is really a quantifiable upgrade. The 3.9:1 rear gears as about as tall or long as you can reliably find. RWD 626s had a 3.63:1 ratio. I feel this would be a massive undertaking as availability is extremely limited. I'm afraid to ask, but why do you need to travel faster than a theoretical 140mph? I would rather have a lower top end with shorter gears for punchy acceleration.

 

Cause when the boost hits that stock rear just doesn't have enough legs. I'm almost in the same boat, but I'm just going to keep the boost low and enjoy the stock rear.

Whats wrong with the ford rear? You can swap diff's on the fly. NA Miata's use the same rear IIRC, but I don't know about their gearing.

 

With an 84-85 bigger rear, the first point of weakness is the centre itself. I have a Guru/Xtreme rotaries torsen centre, which is documented to easily handle well over 400rwhp (~450rwhp on dynojet). A lot of this is due to the inherent strengths of the Torsen design and I imagine OEM torsen swaps (eg from a Miata) will provide a similarly strong centre, provided it is assembled well. During assembly, there are little mods that can be done to strengthenthe stock centre, such as replacing the collapsible pinion spacer with a solid one, see eg this discussion: S3 rx7 diff. Stronger axles? - AusRotary

The next point of weakness is the axles, but the 26 spline axles can still handle a fair bit of power. As to how much power comes down to driving habits and the amount of traction you can get at the tyres. I know of a car that ran 26 splines axles reliably into the 10s. It was only once the switch was made to racing slicks and hard launches and low 10 sec 1/4 mile runs that the axles couldn't cope anymore and he was replacing them regularly. The safe limit for the thicker axles seems to be around 400rwhp - beyond this, you run much greater risk of failure.