i recently put a set of ElectroHarmonix tubes in my silverface champ (12ax7 and 6V6). they're a good bit "gainier" than the (original) RCA/GE tubes that were in there before.
for some things, the earlier breakup is nice. for other things (like that compressed, chimey clean "typical fender" sound), i prefer the original tubes. i'm gonna pick up a set of the higher headroom groove tubes and give those a try for the cleaner stuff b/c i really want to retire the original tubes.
i guess ultimately the "upgrade factor" will be determined by you--what you hear and do/don't like about the current tubes, what characteristics do you want more or less of, that sort of thing. different tubes are a relatively cheap way of changing the character and feel of an amp......and unfortunately it's kinda hard to say what an "upgrade" will be until you've heard it.
cheers,
wade