Generally it's the torque that kills 5 speeders, even the later TY54 models. I was running 401lbft of torque on one, in my own car which I never launched, accelerating in 3rd at about 70% throttle and doing 60-70 mph and stripped 3rd gear.
You will find that non DCCD 6 speed boxes are cheaper. I recently sourced a non DCCD 6 speed box from an 02 wrx with prop and diff, linkage etc for £1,500.
Cheers
Alistair