I went ahead and tried a skirmish match, and the AI was, indeed, noticeably better. Not quite good enough to win, but I did, at least, face some opposition. That gave me the confidence to try the Galactic War on hard mode. I think it might still be too easy. I was able, with barely any technology, to knock out one of the three opposing AI leaders. Since the whole premise of the match mode is that the AI leaders are tough fights where you have to build up your technology to even have a chance, that suggests to me that the difficulty still needs some tuning.
The real question is whether I'm going to bother. It's likely that I won't finish my currently galactic war before the end of the 20 hours (although, given how easily the first AI fell, maybe I will), and so trying the game in "relentless" mode seems like a bit of a superfluous cap to my Planetary Annihilation experience.
Then again, maybe the game will surprise me. I feel like I have it all figured out, and unlike, say, Starcraft, it doesn't have asymmetrical factions or a complex (or, indeed, any) storyline. I've been having fun, but there is no doubt that Planetary Annihilation is a simple game, and thus it probably doesn't have the longevity to become a serious obsession.
At least, not in single player. I understand that the real meat of this type of game is in online multiplayer, and I should probably give it a try, just to get the full experience, but let's get real. I would get creamed. At the end of a match, it gives you a breakdown of your various stats, and in the most important RTS statistic - commands per minute - I'm abysmal. I believe I topped out at 10. To put this in perspective, a professional gamer can do 300 or more.
I may well try a multiplayer match, just for the sense of perspective, but I'm almost certain that it will be a quick and humiliating defeat.