my 550's are maxed out at 18 psi

[niterydr]

Hacking the mas allows more air to pass through uncounted. It is a temp fix, and works, but the better solution is to upgrade to something that can count the incoming air. The factory 1g mas doesn't have the capibility versus the 2g mas. The 2g mas has a larger area to pass air through, thats why you have to compensate for the larger size. With having 550's, you would be better off upgrading to a 2g mas instead of hacking the 1g mas and having to take 15%+ percent out of the fuel curve.
IDC's are important, but you must remember if you don't correct the signal, the computer will use them like factory. When you take away signal (- on the s-afc) you are actually indirectly taking away fuel, thus lowering your injector duty cycles. If you are to far off the 'good curve' you can knock. This means you can knock when you are rich, or lean.
The hacked 1g mas is a proven way to go, don't get me wrong, but I personally like using the 2g mas. Accuracy sometimes costs more, but is better in the end.