We can fix the WHM plugin if can get access to a FreeBSD server to play with. cPanel + FreeBSD is not popular.
As for native build for 7.0 and 8.0, I am not sure. We already got 3 packages for freebsd, while only two packages for Linux, if support 7.0 and 8.0, need to add 4 packages, just too much.
I think the 6.x compatible binary still be able to take advantage of the kernel improvements in 7.0 and 8.0. Just the userland libraries are different, the kernel is still run at full speed, maybe there is a thin kernel call translation layer, but the impact should be minimum, just like running 32bit application on 64bit OS.
We will do some performance test see if there is any major difference.