I have to admit that I gave up with fission and fusion reactors - the lag caused by the mass of turbines and cabling was just too much for my poor server (2GB normally 4-6 players). Since switching the chunk-loaded shared power station over to using a pile of MFR's biofuel generators (and automated farms for fuel production), it has been coping admirably. The other thing we really had trouble with was MFFS's field projectors, having a force field up made the server very unresponsive, and caused it to fall over regularly with large-ish fields. The interdiction matrix seems fine though. Also, you mentioned that players are roaming about a lot. Generating new chunks can be pretty taxing on the CPU.
I'm using 1.3.9, and managed to get MCPC+ installed without problems using the instructions here in the forums. The NoLagg bukkit plugin looks like it might be useful, and it sounds like there are a fair amount of other plugins that focus on this area. If you don't want to mess with the server too much though, I think there is a forge mod called aPerf that can do come performance tweaks. I was seriously considering trying some lag reduction mods/plugins, but since ditching the giant reactors and force fields, no one has had any problems really. In fact, I've been happily adding extra functionality through plugins.