Ah OK. Yeah, I don't think a GFA would be allowed in Production. Under Prohibit Modifications for grips is says no to any addition or removal of material which changes the factory profile or adds function such as a beavertail or thumb rest. But I didn't see anything in the rules that would prohibit it in Limited or L-10.
For IDPA's SSP, I think this would fall under the Excluded Modifications of an externally visible modification... but I think it'd be OK for ESP.
Which is too bad because I sometimes get bit by the slide of a Glock due to high grip/thumbs forward. Curiously enough, I didn't get dinged at the Glock's Match, and my gun had 300 or so chances that day to do that to me...
Something else to put into the blender: Do you want a gun that you can shoot in IDPA as well as USPSA, 3-Gun, and Steel Challenge? IDPA is the probably the biggest limiting factor, so if you do you might consider getting the Eagle / short dust cover 2011 frame and a bushing barrel. if you need/want more weight out front, you can always get a tungsten guide rod. I'm keeping that option open, but haven't felt the need to do it yet.