the basic question is how do you change your tune to take advantage of the larger catless downpipe.
Well, given that it was actually the downpipe that was the major restriction (I guess that you evaluated this before the purchase?) I also guess that you had issues with high back pressure and high EGT's at high loads and that you concluded that the major restriction wasn't the turbine housing nor the exhaust manifold and as a result invested in a larger downpipe? If so, measure what kind of back pressure and EGT's you have now, check that the compressor outlet temperature increases linear with boost (and not exponential, suggesting that the compressor is running out of steam) and just crank up the boost, adjust the fueling to keep EGT in control and adjust timing to avoid too much timing pull...
Again, I really hope that you concluded that the downpipe was the major restriction... Otherwise there is not much power to be gained until you remove the real obstacle (which might be something completely different).
