Have the mouse plugged into my PC at home, got it all configured up (looks like 900dpi is just right for me) with the latest firmware and everything. Mousewheel feels just fine. I'm afraid this just may be an OS X issue. I have access to two Macs here, one with Leopard one with Snow Leopard, will do some testing with the mouse this weekend.
Have to say it felt great for the 10 minutes of StarCraft I just played (just did a micro challenge). It will definitely take a little time adjusting but I'm fairly confident that I will feel some direct benefit fairly quickly. Micro almost felt easier/faster right off the bat. I never knew how much I was struggling!
The scary thing is now I have to buy one for work...oy. Now I'm going to have 3 Lachesis sitting around not being used. I can try to sell the 5700, though, it's only ~6 months old and probably ~100 hours of use.