I have a "built" 4L60 that is supposed to be able to take 700hp/tq. Car is a forged ls1 on 10psi of boost from a D1SC Procharger. Car will shift like butter when driving normally. At WOT it will just rev out and hit the limiter and not shift. From a roll it will downshift to 2nd as it should then rev out and pull strong right to the rev limiter but wont shift to 3rd. Every once in a great while it will shift but out of 5-6 attempts last night it only shifted once. Is this a mechanical issue with the trans or is this a tune issue? I bought the car with this trans in it and plan on a TH400 this winter anyway. Thoughts?

 

could be either mechanical or tune
I'd start by making sure the fluid level is correct...something 99% of people never do properly
Then I'd take a log and watch when the shift is commanded. Having to "lead" the shift is not uncommon at all. Especially for a blower car that will climb rpm quickly. You may have to trigger the shift at 5400 to make it by 6000. Sometimes more, sometimes less.
You have to trigger a solenoid, that solenoid has to seal, then create pressure to stroke a valve, then once the valve is stroked the oil has to move all the way from that valve, to the clutch you're trying to apply. Filling the whole circuit. Then, you have to actually clamp the clutch.
All that takes time. If that all takes .2 seconds. you'll need to lead it by that much