I have the GT28R with 0.42 compressor (yes the small one) running no dump valve chatter is great, have had no problems at all and have been running for a year, even PE 400BHP focus runs no dump valve with the GT28R (0.60 compressor) i now have the GT2860RS (.60 compressor, and 0.84 turbine) and have no plans to put a BOV on either.

High powered jap cars run GT series turbos with out BOV, no matter the bearing turbo manufactures dont recommend running with out a BOV as the compressor will go into surge when you close the throttle butterfly and can cause premature turbo failure, but the most damaging surge is when the turbo compressor is too big to spool up on a small engine.

although chatter isn't recommended it sounds better then any other BOV ive heard