Thought I'd contribute a bit, in general at least. I run a similar setup to a few people here (6350 @ 4.0 ghz, 16GB ram (3.5 allocated), 280x), and find the game... unplayable without Optifine. I get that same unusual nearly 40 FPS running a modded pack , which - for me at least, is pretty unacceptable. The second I throw in Optifine (especially the U edition which allows for Multicore support), I can get all the way up to those 200 / 300 numbers, though not nearly stable enough to warrant the jet engine noise so I just cap it at 70-80. Fastcraft can add a bit to that as well, mostly stabilizing the frames. As for other sources of lag, my Win7 installation is probably quite 'stale' by now but I've got no other issues with anything else. Also, I'm running MC and my OS off an SSD, not that it means anything.
Things like fastmath and fastrender (this moreso, for somewhat obvious reasons), actually decrease my FPS.
Mipmap levels seem to have an usually large impact on my FPS.
Dropping my view distance down can dramatically increase my FPS. I play on '8'.
I decided to try optifine and even that did not make much of a difference in my case, though I can't say it did nothing. On the whole, with a lot of toying with settings, the best I was able to come up with was ~10fps boost from optifine, which really isn't enough. At this point, I've disabled mipmaps and also have my render distance set to 8. I tried every combination of fast render/math on/off but they were all effectively the same.
Per suggestion from @Wagon153 I tried setting up a linux installation and running the pack from there, but that too provided similar results, which I was surprised about. I did get slight gains on linux if I dropped the FOV down to "normal" (I usually play with FOV around 95 and I hadn't given this much thought previously) of ~10fps, which I was not able to duplicate in Windows.
I'm at a loss, so I guess I'll live with it for now, but I have no clue why it runs so poorly for me overall. I guess I need to just be grateful that I do have something better than a potato to run things on, even if it barely outperforms them.