I also fitted a set of the yella terra ST2027's to my EL XR8. I can't tell you or a power increase on their own, as i fitted them at the same time i had the heads off for some porting.
Real easy to fit, and i didn't have any problems with fouling with anything else like rocker covers.
As far as price goes, i rang a few places when i got them, and they were cheapest if you brought direct from yella terra (in morrabbin if i remember correctly). One thing you might want to check if you buy from yella terra direct is to see if they have any "factory seconds" that Ford rejected. You see, these were the same rockers that were fitted to the later model XR8's, and Ford apparently had a pretty strict quality control policy. I think the set i got (which they mixed and matched from a few reject sets to get a good complete set) only cost me $360, compared to the $540 for 'perfect' set. I got these back in OCtober 02, and have had no problems with them yet (hopefully never). I don't know if these reject sets are still available, but if you're in the area, it might save you a few dollars.