Not quite true, we gained around 400rpm spool up time when testing our twin wastegate manifolds back to back against an HKS manifold. 400rpm is a lot on the road or track, this is why we decided to stop production of our single wastegate kit and just sell the twin version.
HKS and Greddy manifolds both crack over time as well, we cracked a few back in our early days of Time Attacking, part of what prompted us to make our turbo kits.
We've never had one of our crack in 7 years of producing them which is pretty good going compared to other brands out there.
I agree with Precision, any problems and you're in trouble. Borg Warner and Garrett have plenty of support in the UK, not that they tend to go wrong much.
If you are using Garrett make sure you upgrade the bearings, Owen Developments do some nice upgrades for them.
Garrett turbine housings are like chocolate as well, again back in our Time Attack days the housings cracked, warped and fell apart under the abuse we gave them. Switched to Borg Warner and never had a problem again.
I think there are some nice stainless steel housings available for the Garretts now, definately worth doing if you want them to last.