I hate to say this but unless something is done to fathom out the lag issues, putting all the mods into one pack is not going to work and will just die horribly in a server environment.
Reading the other posts, watching the videos, putting 2 and 2 together, it seems there are multiple issues with world gen/chunk drawing. Some of the trees in XL Biomes (redwoods and the other tall trees) cause chunk drawing/FPS issues and have done since beta A, but you add all the other mods in and the redwood problem is multiplied. Go into the water and there's an issue there with what seem to be air pockets deep down under the water, these appear to be single blocks of air but have found some 2x2x2, I'm not sure what causes these but have experienced them in Tekkit so it's nothing new, just again more mods, more things to draw seem to make the problem worse.
Then there's the ores, holy mother of god how many ores do we need before there's no cobble left and to what purpose do half of these ores serve? It would appear a good lot of them are just not used or used by xycraft for decorative purposes.
Did anyone stop to think that putting less commonly used ores like Xycraft in the Nether would be a good idea? This would relieve some pressure on the OW and again on the chunk drawing.
Ultimate is about to become the ultimate flop unless care is taken.
Anyway not wishing to start another Gregtech argument going but Ultimate pack will always be Semi-Ultimate IN MY EYES without Gregtech
bunch of wimps