FWIW, I came to the same conclusion and ended up building a machine. I'm running an X79 board, 3930K Hex Core proc, 16gb PC 1600, GTX 670 hitting 20K+ Geekbench and I have less than $2k in it WITH a 27" IPS LED display (Korean panel from eBay, same panel in 27"
screens/iMacs). ML is rock solid, never a single KP. It's beyond what a Mac Pro twice the cost will perform at and certainly the new max config iMacs.
It is not for the faint of heart and it's not going to be a vanilla install (requires some tweaking and compromises..ie. no machine sleep, no speedstep) but it's also been a very fun project. I'm overclocked to 3.8ghz (4.2ghz in win 7 where I get 23K geekbench!).. those are very conservative overclocks and beyond easy to configure with these boards. Even at stock speed she pulls over 16K.
The thread that got me going:
X79