Errrrr, Allocating more RAM to FTB just allocates more RAM to the JVM (Java) - which does the exact same thing. I wouldn't recommend setting a global JVM memory limit as applications usually handle this quite well, but since FTB doesn't, they give you a pretty little slider for it since there's so much more content.
well... the aura thing does still have merit tho. especially if there are entities being spawned in the area that do affect it directly. wisps being spawned might do this perhaps? I know that if you mess up the aura of an area wisps will start to spawn there until the aura has balanced out again, so these dungeons might have an impact on the local aura, thus causing a lot of recalculations. even tho the server might not lag noticeably, does not mean it wont be noticed by a client with a slow connection. (lots of updates has to be pushed to the client after all)not sure if this is a possibility or not but savanna biomes have those wisp dungeons in them and i notice that there is one at the mindcrack spawn...