Comparing the EFR6258 (38lb/min) and the GTX2867 (47lb/min) isn't really fair to either turbo. AFAIK
Garrett doesn't make a GTX that's comparable to the EFR6258 - the closest you get is the GT2860RS (37lb/min), but the EFR6258 will spool/respond significantly faster. I haven't seen any GTX28 twin-scroll housings out yet, but I could be wrong?
Ceramic is a pain in the ***. I would go with stainless and forget the coatings.
Don't use anything but a factory MLS 94-00 head gasket, and ignore anyone who says otherwise. Squish matters, and if you screw with the HG height you screw with the squish.
Don't bother porting or gutting a factory IM. Have someone build you a custom sheetmetal unit (ARtech?) or put together a hybrid Skunk2/Blox B18 manifold. I have the hybrid B18 piece and it is worth a bunch of power above 5000rpm with very minimal gains under that.
FYI, we're releasing a turbo kit in a few months that will be pretty much exactly what you are looking for - small-volume tubular manifold, EFR6258, 3" stainless downpipe, track-tested parts. The standard kit components will do 300whp without a problem.