The problem with the Holsets are the compressor turbines. they designed for diesels that dont rev high so flow for high rpm petrol engines is poor
thankfully Holset have recognised that their turbos are being used more and more in the tuner market and are releasing a "super" version of their turbos with different compressor turbines. they will be a little more expensive but i would def hold off if you havent bought one already
we will have theses available when launched