Your stock blow off valve leaks
I also posted this on the Facebook page but I know some of the guys on don't use Facebook, so here you go.
Not sure why I haven't heard anyone mention this yet, but there is a major downside to keeping the stock compressor bypass valve. It is designed to start bleeding off boost pressure under high boost. If you are raising the boost by compressor mod or SC gears, this will be affected by your stock bypass valve. You can see a bleed hole that leads to the diaphragm that will help it start opening under heavy boost conditions. This is common on many stock forced induction engines like like the Subaru WRX. By changing to an aftermarket valve, you can eliminate this boost bleed at high pressures. I'm curious if any of the tuners have messed around with changing them to see exactly how this changes the dyno graph.
Ride your own ride