Found the quote I was talking about, it's by MrWizard himself:
You can read about the topic here in the discussion it took place last year when the decision to ban PS3 wireless pads was made:
http://www.shoryuken.com/forum/index.php?threads/reminder-wireless-ps3-stock-and-wireless-joysticks-are-banned.133258/
In my opinion, you can try to get the modded PS3 pad but you run a high risk of the judges forbidding you from using it at all. I think you should give the PS2 pad a try, it is really not so different from the PS3 model. Though, maybe I'm biased because I've been playing on Playstation's pad since forever and got accustomed to all three iterations.
The decision is ultimately yours. But if in the end you decide to go with a PS2 via a converter, make sure it is lagless.