Just found this handy piece of info from
www.wrx.com.au, wheather it be true or not is another matter.
"Ben, the VF34/35 was designed between Subaru and IHI as the Homologated turbo for the group N/PWRC cars, and has been designed to run optimally with a 34mm restrictor. This results in the turbo running out of flow over about 5500/6000rpm (about where the restricted engines rev to) and going into a surge condition."
Reason why the 34/35 get compressor surge.