Would the HM67 chipset be a bit more efficient than H67, or would the difference be pretty insignificant?
TDP of the H67 is 6.1W vs. 3.9W for the HM67. Source
What about the CPU's - would they actually be more more efficient than similar desktop counterparts? They're clocked quite a bit slower, in general, but Turbo jumps up almost to the higher-end desktop CPU levels.
The chip is the same, so clock for clock mobile and desktop parts should use equal amounts of power. The VRMs for the mobile parts are smaller since the boards only support CPUs with a max. TDP of 45W (for the quad core) ,or less likely 55W (for the quad core extreme parts), compared to 95W for desktop quad core.
Between lower clocks and smaller VRMs there should be a few watts of power savings.
What about the heatsink? I have no clue what hole pattern that Jetway board uses (will try to find out), but something proprietary might be a problem.
The mounting holes for socket G2 are 51x51mm. Coolermaster and Cooljag offer coolers, but they are similar to active 2U coolers: small aluminum or copper heatsinks with a 60mm fan on top.
Tom's Hardware ran an article about Ivy Bridge CPUs being compatible with H67 (and other) boards... Would it be safe to assume that the mobile chips will also be compatible with HM67?
No, it is not (ever) safe to assume. While desktop Ivy Bridge CPUs can electrically work in mainboards with a 6 series chipsets they must be supported by the bios. The bios size has to be at least 4MB, if memory serves. It has to hold microcode updates for both Sandy Bridge and Ivy Bridge CPUs.
Iff the bios is big enough and iff the board manufacturer supplies and update, it will work. (if and only if)
If a fairly big heatsink can actually be mounted on this board, then could the mobile CPU be cooled passively more easily than a desktop counterpart?
Mobile parts have a lower TDP, so yes. The mounting hardware doesn't fit a desktop heatsink, so no.
GPU mounting holes are similar, Ati 3800 through Amd 6800 have a 53x53mm pattern, nVidia 460/560 mounting holes are 51x61mm. With some modding even an older GPU cooler like the Zalman VF900 or the newer Deepcool V400 would provide superior cooling at a lower noise level. For passive operation the Deepcool V400 might do, maybe a modded Arctic Cooling S1 would work.
The Thermalright HR-03 mounts with a bracket that is not attached to it, so in theory it should be easiest to mod. It is big and it has a weird shape for a CPU cooler.
I think the base of a big desktop CPU cooler is too big to fit 51x51mm mounting holes. Maybe the base of the Xigmatek Gaia with 3 heatpipes is small enough?
I've had my eye on that board and for the same reason, too.