You are correct. They started at 310HP in the early 70s with a compression of 10 or so, and went up to 330HP with the European GTS by 1973, at a compression of about 11.5. The HP was then reduced when the compression was reduced to 9 someting and the emissionins crap was put on. It was down to about 275HP. This all happened in th mid 70s, which when combined with the gas crisis ended up killing sales.
Usually owners replace the carburator and exhaust to gain an additional 15 HP or so. The original ones were very restrictive. Mine has both replaced as well as a rebuilt engine that has been 30 bored over and high compression, but not sure how much. It has TRW forged pistons, a performance cam, and new heads with roller rockers. I haven't had it dyno'd yet but my guess is I'm probably running around 360HP. Not much by today's standards but still fun.
Of course you have the guys that put monster 427s in it:
http://www.youtube.com/watch?v=Vlen0A-Cmss