My setup:
ASUS Maximus III GENE LGA 1156 Intel P55 Micro ATX Intel Motherboard
Intel Core i7-860 Quad Core CPU (95W) 2.8GHz (Hyper-threading, Turbo Boost max 3.46Ghz)
Mushkin 8GB (2 x 4GB) DDR3 1333
Antec Solo Case
Thermalright XP-120 Heatsink with Arctic Cooling 12 fan (Rated at: 56.3 CFM / 1500 RPM / 23.5 dBa).
As I suspected, the back piece of this kit from Enzotech worked fine with my xp-120 LGA 775 version heatsink. Instead of using the piece from the xp-120 that goes on the bottom of the motherboard I used the piece from the Enzotech kit. I had no problems installing the heatsink onto the LGA 1156 socket, and the pressure of the heatsink on the CPU seems fine. Also, in regards to the top pieces that go onto the top of motherboard which the heatsink attaches to, they only fit one way so it's not too difficult to figure out. You'll need to install the CPU before attaching those clips to the board.
I realize I could have spent ~$50 and gotten a newer, and maybe even better, heatsink made for 1156, but this seems to be a fine solution. Also, I prefer the downdraft (downward blowing) heatsinks because it helps to cool the other motherboard components as well. I might eventually buy the newer Thermalright AXP-140 RT, but I can't find it in stock anywhere, and using my existing heatsink is cheaper
RESULTS of Thermalright XP-120 on LGA 1156 Motherboard:
Here are my temperature results according to "Real Temp GT 3.60" which supposedly reads the TJunction (cpu core) temperature... (Not sure what the room temp is)
Idle: ~24 C to ~35 C
Normal usage: ~35 C to ~51 C
Video Encoding with Adobe Media Encoder CS4 (this is about the most intense real-life use of my CPU), uses all 4-cores at 2.93 Ghz (~75% to ~100% load): ~57 C to ~73 C max
Prime95 torture test: Maximum temp I saw while running this test was ~78 C - which is about the max safe TJ (thermal junction) temperature recommended by intel. Prime95 on all 4 cores runs at 2.93 Ghz and limiting it to just 1 core it runs at 3.46GHz (Thanks to intel Turbo Boost). I'm not sure at which temperature these CPU's are designed to throttle at (reduce the core speed) but the CPU never throttled while running Prime95, so this is a good sign that this XP-120 heatsink is sufficient to cool the i7-860. I suppose where you might run into over-heating issues is using this on an i7-900 series processor rated at 130W.
I figured this information might be beneficial to others considering that in my online searching others were asking this same question about the XP-120 on LGA 1156 but no one had said whether or not they tried it and what the results were. I may also post video/pictures in the future.
Did I mention, it's also very quiet with the fan I'm using.