Another thing I've looked at to reduce backpressure is headers and downpipe/wastegate. Equal length headers are supposed to be superior for turbo efficiency over log style headers because it reduces turbulence from the various pulses. Maybe a Kenne Bell/ATR style header with a collector near the turbo would be a better design. Another thing is a smooth transition coming off the turbine to the downpipe rather than opening to a big step. A downpipe with no wastegate provision may be the best solution. Also, for an external wastegate setup, it will be better for turbo efficiency to dump to atmosphere rather than reintroduce the air back into the downpipe.
It may be very small gains, if any, for these but it may be worth a try. I may have access to all of these to try them except that the downpipe is only 3" vs my current 3.5". It may not matter, but if its only a miniscule loss going to the 3" it may wipe out any gains.