Dont know about uprating 5 speed but my weakest link was the clutch, It just kept slipping under load. I had a Techniclutch 450BHP rated fast road cluth cost £180 (Absolutely rubbish by the way) slipped from day 1 to failure at 500miles or so, Only ran 0.4 bar boost too....
In heinseight should have bought RPS MAx for about £450 Rated to 550BHP, Speak to Michel Lane or Kevin at MVP for this.
I have heard of the W58 reguarly taking 500+BHP aparantely what happens is when they do go they start to become hard to engage 1st gear, So they do not suddenly blow up...
I went straight up to 600BHP+ so decided the only way to go was bigger flywheel and 6 speed TT style 900+ BHP capable:D
Cost me over £4K so far to fit myself:(
Knightrider