Which kind of racing you are about? On drag racing first to go is diff in my past FWD experience. Mentioned quaife is suggested step(or factory Q2, but its probably slightly weaker unit).
I broke 3rd gear on strenghtened integrale gearbox(housing and bearings, and cryothreaded as bonus) on fast "street" driving.
There exist a group N kit from italy (multiple manufacturers), which consist input shaft and first 3 gears. Its simple swap for OEM parts, but its designed for integrale and with little more work also on 16vt or early 20vt 5 speed.
I used them to rebuild my box, but not much drive it yet due engine went down(never ending story).
Cost around 1500-2000eur dealer depend and no claimed torque capacity figures.
Maybe better to go for dogbox if pure race car, but its almost triple price.
Last to mention, some harsh clutch will not help either. I used tripple palte sintered APracing and gear broken on low boost just after shift.