I take it when you mean you've just got it on the road that it's a fresh conversion, ie you've not driven it before?

If that's the case - what chip are you using? I think I had the same problem as you on my old Ka that I converted to RST power. *Just* before coming on boost (so around 0 vacuum and 0 psi) it would feel like it didn't want to go, but then the tiniest hint of + pressure and it was off like a rocket. I swore it was the chip I was using but never got round to changing it to see if the problem went away. Might be worth a shot.