Thanks for adding the option, looking forward to a new download!
And yeah, there's a lot of overhead on FAT - basically it seems to be as slow as if you manually wrote zeroes to the file until it had the target size, whereas on NTFS it's a few minor FS meta-info manipulations.
It's worth noting that, with the "old" method of writing files, you don't need a heavily fragmented filesystem just to get a pretty fragmented file, so it's nice to that you've added support for space reservation.
As for defraggers, Nuts & Bolts used to be great in the days of FAT and Win9x, and none of the NTFS defraggers are quite as good. PerfectDisk is the only one that seems to be much of a difference from the one that ships with 2k/XP.