Dredging this up from months ago to say the exact same thing.
It strikes me as very very strange that a platform as robust as the Technic Launcher doesn't give the user control over JVM arguments, when even the vanilla Minecraft launcher does.
There are some optimizations you can throw in as arguments which make a night and day difference in how a modpack laden Minecraft runs, and Technic doesn't give access to them. On behalf of myself and other users who know what they're doing, we'd really like the option to be able to tweak our settings a bit more than the minimal "maximum RAM" option we're given now.