Are there any benefits to deleting unused Mac OS X kernel extensions?
Actually, any difference in boot up time would be negligible - OS X doesn't boot /mach_kernel — it boots the kernel cache (/System/Library/Caches/com.apple.kext.caches/Startup/kernelcache), which already has the kexts you are using. This gets rebuilt if /System/Library/Extensions gets modified (or if you touch(1) it deliberately). 

In terms of the filesystem space, as of Snow Leopard, everything is compressed. Meaning you're not really wasting too much space, either. 

There is a downside, however — if you delete kexts, even with kext wizard, or whichever utility, there is a remote chance that some kext will be loaded on demand (e.g. via IOKit probing, on some USB device, for example). In this case, a kext might not be found - and you lose a driver.

In short, if it ain't broken, don't fix it. Leave /System/Library/Extensions intact.