The other company in the UK to speak to regarding upgrading the stock 9 turbo are Race Tech in Sheffield. I'm currently in discussion with them about rebuilding/upgrading mine, which appears to be using oil at the moment.

 

The mag comp wheels from the 571 9RS turbos last years+ at 24psi no problems, not recommended above that. The best you'll get if you want spool bur more power than the 16G variants is using the TiAl TD05 turbine (available from any MHI evo turbo with 'TD05-HRA' in its code (the A denoting the TiAl turbine)), and a lightweight billet aluminium compressor. Like a BBK Lite but using a TiAl turbine. A local Aussie turbo shop (GT Pumps) builds them including an upgraded thrust bearing for reliability, killer response and midrange with great topend, 250kw/400nm on pump, ~295kw/450nm on ethanol.