I've been doing some back to back comparisons of standard downpipe, standard downpipe with pre cat removed and an aftermarket 3" DP with sport cat (APR).
I would say don't waste your time with decatting the OEM pipe and go straight to an aftermarket 3 incher. The difference between decatted and catted OEM is so tiny, it's not worth effort imo.
The problem with the OEM pipe isn't so much the pre-cat, but rather the 2.5" bore and the horrific bottle neck where it's crimped to clear the gearbox.