The six inch pads just don't seem to do well on a PC. You really can't get enough pressure on them before the PC starts to bog down. I think you'll be much happier with the four inchers. They will also give you better correcting power.
SIP works much better if you put a little
PO106FF in with it. It'll extend SIP's working time alot, give you a good clear breakdown, and a much finer finish. It also won't "fight" with you so much. Just be careful not to use too much, as it'll start to diminish SIP's cutting power.
As for the total amount of correcting power, you have two things going against you: 1. using a PC instead of a rotary. 2. Dang hard GM paint.
PC's just do not even come close to the correcting power of a rotary, so you have to expect to use *multiple* applications of SIP (or even something stronger) in order to get the paint corrected.
I do use a PC for glazing and LSP'ing occasionally.
As for getting more corrective ability out of the PC, you can, but it's kind of a mixed bag... If you use a stronger correcting polish, you will probably end up with some compounding haze and marks that can then end up being every bit as difficult (if not more so) to remove than the original paint defects. I've never really had much luck using anything stronger than SIP to correct with the PC. Lot's of guys have, though, so hopefully they'll chime in here with the proper polish/technique to use.
I wouldn't bother with using a yellow pad on the pc. They are just too agressive to use with the PC.
My two cents, anyway.