The T5's do not use a T25 at all - all the Turbos are Mitsubishi except the new S60/V70 R which are KKK turbos.
With a basic remap and air filter costing £600 you can get 280bhp from 850's and 310bhp from S/V70s - even more after that but its not cost effective really. Deffo recommend one, even over a Saff.