I think you might be giving in too much to marketing hyperbole here that 64-bits is better than 32-bits. That will probably be true in 2020 once we have machines with 64GB of RAM or more, but it ain't true today. 32-bit is better today on current hardware with Mac OS X if you're looking to eek out all the performance you can, except it specific circumstances where you need direct access to more than 4GB of RAM for an application.
The current MacBook Air is limited to 2GB RAM. The main advantages of the OS X 10.6 64-bit kernel are the ability to address more than 32GB of RAM (32-bit kernel using PAE can only use 32GB of RAM) and direct address space more than 4GB of RAM for an application without using PAE.
And, because of the nifty 64-bit architecture in Mac OS X 10.6, even with a 32-bit kernal, under 10.6 with a MacBook Air you still run all 64-bit applications and the OS itself is 64-bit. So it's not like there's some 64-bit OS application or other 64-bit application you can't run.
If someone wrote a 64-bit EFI for the MacBook Air (to allow use of the 64-bit kernel), for programs that run within the 2GB of RAM, they'd run just a little bit slower. Not much, but just a little, because 64-bit is slower because it takes a little longer for the hardware to physically manipulate 64-bit data (8 octects) than it does 32-bit data (4 octets). Here's an example of the speed difference:
"Benchmark: 64bit vs 32bit, Page 3" The conclusion:
Now under that 64-bit kernel/EFI if you got a MacBook Air and somehow shoehorned 8GB of RAM or more, and ran large-scale 64-bit apps (of which there are currently few) that used all the memory, that app should run just a little bit faster. Why? Because instead of using PAE, you'd get native address space access to that 8GB of RAM and might be able to get faster performance under 64-bit. Or maybe not, it's going to depend on the app, 64-bit might STILL be slower.
Now, take the hypothetical to the limit, if that 64-bit EFI/kernel MacBook Air had 64GB of RAM... well then that's something that the 32-bit EFI/kernel can't do because it's limited to 32GB of RAM.
So bottom line, forget about the 64-bit kernel hype and live with a faster MacBook Air. Unless, of course, you've modified the hardware to have more than 32GB of RAM, in which case that 64-bit EFI/kernel is going to come in handy.
Just a note, Windows is a different story. Windows 32-bit kernels can't even appropriately handle more than 3GB of RAM and can't even get to 4GB of RAM without using PAE. So once you have more than 3GB of RAM with Windows, you kind-of need to move to a 64-bit kernel or you're screwed. Another reason why Mac OS X is better than Windows.