I feel like with few exceptions, everyone that runs a Holset stays around 400hp no matter how high the PSI. They sound amazing on the internet, but they just don't seem to deliver in real life. I'm not so confident you'll be happy going from a 20g to one of those.
Has anyone here made big numbers on a Holset? Maybe it's just all the kids on tuners bolting these on without using their heads..cranking the boost to 40psi and paying someone to keep it from popping.