...75 boilers with a total of 1,350 engines...
Wow ! Would be / would have been very cool to see !
Currently, I'm at 10 boilers and 180 engines and that has suited my needs just right. I can run 3x MFR lasers at full speed or else 2x MFR lasers + lots of ExtraBees when I'm in bee-breeding mode. I have 20x TE RECs acting as MJ buffers. They don't seem to cause any lag at all. I have the boilers and engines in a separate "power room" along with my big fuel tanks. It causes some client lag, but not too bad.
I've done "big base" builds before, but this time around, I used MystCraft to keep my areas separate and that has helped tremendously.
I bet I could get 1000+ engines without too many server or client issues if I limited them, to say, 100 to 200 engines per client-loaded area, which means spread out across 5 to 10 sites, but that would defeat the purpose of impressive "scale". Might as well build 1 for show and PowerConvert for power beyond that.