When I replaced my turbo about 2 years ago I shopped around and also spoke to a lot of workshops with Dynos.
My general findings were that ATS could make a highflow turbo that could handel 14psi without stuffing up, but that they didnt produce any more power that a standard turbo at the same boost (and in mant cases workshops reported small to medium drops in power)
The only garanteed success story was GCG whost turbos made more power than standard at any given boost, were reliable at over 14psi, and came on early (nearly the same as a standard turbo, which was another problem with other highflows which were laggy)
I went for GCG and am happy with the results, my only other option was to look at an aftermarket turbu HKS2530 etc.