Agreed, the increased render distance and multitude of options is awesome.
Its something I started doing on 1.2.5, perhaps the newest unified 1.3+ is better at handling entities, but still no dual or multicore support (except for chunk loading care of optifine)
The real boost comes with using more than one core on slower machines, as I said im only using a 2.3ghz core, so its pegged at 100% on one core just doing rendering, with a server it pegs one core at a few percent until i get alot of entities, such as lots of items in pipes (logistics) major tree farms (think multiple large jungle trees being felled) or a mining laser (hundreds of blocks on the ground)
Perhaps an i7 is a bit less prone to the overhead of many entities, but im sure that having one core doing all the logistics of world ticks and the other doing rendering has to help, Im just not sure it would be noticeable when your already getting great performance.
My desktop i5 handles things alot better, its 3.1ghz with a 7970 and Ive not had any issues, I mainly play on my notebook i5 @ 2.3ghz with a nv 315 which is borderline with modded minecraft installs.