Culture is below Xeelee. Very very much below Xeelee. So not quite the top.
Ehhh.
The Culture in "peaceful non-scary benevolent ubertech group" mode would only have difficulties with the Xeelee use of tactical time loops/retrocausal computation.
Gridfire is every bit as terrifying as a Starbreaker, and of course we're probably assuming that the Xeelee didn't show up in the new universe, set up an anti-history loop to insert themselves at the beginning of time so they already have all the tech and science and everything figured out, right?
The few times the Exultant group really got clean wins was when they were able to pull shit literally out of nowhere (like the Alcubierre shielding trick) and Gridfire is like the ultimate "fuck that guy, and everyone around them, and good luck hitting me back because I'm
behind 7 proxies light years away" toy.
A GSV or ROU or whatnot isn't as fast as a Nightfighter going flat out, but they have "reach out and fuck your shit up" capabilities way beyond anything outside of some Ghost fuckery turned up to 11.
Probably wouldn't get much luck trying to effectorize a Xeelee, but simply bathing them in Gridfire and zooped in CAM munitions until it falls apart is gonna be a heck of a thing to fight against.
The Photino Birds made use of the whole "we can hit you but you can't hit us as well" thing with gravity based weapons, dark matter kinetic strikes, and "unbelievable sorts of weaponry" only hinted at in universe.
Full out the Xeelee as a whole decide to stomp the Culture's shit in, shit is getting stomped in, but in a typical sort of engagement where the Xeelee try to swat away an annoying insect? I'd expect the Anti-Xeelee would have to take note and not be quite as amused as it usually seems.
Though last time I was in a discussion where those two got brought up, a wildcard ended up getting brought in: The Shrike.
Which led to various discussions over how the Shrike is way faster on the spot, but not actually faster over a long distance, so it could probably evade a Nightfighter up close, but never quite get entirely away, leading to one of my favorite forum quotes ever.
"So Yakety Sax. With intermittent supernovas." ~alguLoD