I set the render distance to short, and the... uhh... something setting to max FPS, on top of that I set most of my graphics settings to the lowest setting, AND I have a very powerful Mac. I know I would get perfect FPS if I removed my 128x Sphax, but tekkit looks so much better with it. If it helps, my Mac is 32 bit, and my Java version is 6. I am now getting a constant 1-3 FPS, and it does not ever improve, I just find it illogical that any modpack, no matter how many memory leaks it might have, can drop someone from 100+ FPS to 1-3 FPS.
Edit: I am using an extremely good graphics card as well, so I know it's not that. Also, I disabled MAtmos, and at first it made a monster of a difference, but after a while I went back to the same 1-3 FPS