Will the mapping need to be changed to counter the loss of back pressure? If so, who will do it?
Yup, it will. Closed Loop systems can sometimes automatically adjust fuelling by as much +/-15-20%, which should be enough, but remember that 15-20% is against the target AFR's, ie: the mapped target values locked into the ECU. If the map is fundamentally weak, then that's still the Closed Loop's starting point. Ideally it might also need timing tweaks, but that's less likely.
When the pipes are finally fitted, I will have to find a dyno at an S&S specialist to have it checked. Just look back at our French colleague whose de-cat'd and early-piped (noisier) car dyno'd some 25bhp more than ours....
