I suspect your short manifold is causing most of your problem. The longer your manifold the more bottom you'll get out of a given pipe. I had the same problem and same symptoms you descibed. Quite annoying. I threw the longest manifold I could find onto the same pipe and it got rid of the "turbo lag". I did some testing with different pipes and manifolds and it transforms the engine different ways... more bottom, more top end, less bottom, more mileage etc. My two cents.