I'm no expert on this but the overheads you are seeing seem way to big. I formatted an external USB drive connected to the NAS using Frontview and chose ext3, but I was only see a few 100MB loss of space.
I have found that the best way to format a drive for use with the NV+ is to use Frontview, as follows (from memory as I don't have a drive hooked up at the moment):
1) login as administrator
2) Connect your drive
3) Go to 'volumes' and 'USB storage'
4) Your drive should show up with a drop-down menu next to it
5) Select format ext3 and let it do it's thing
Hope this helps.
2 x 2.4 GHz Quad-Core Intel Xeon
14GB DDR3 RAM
Mac OSX 10.8.3
ATI Radeon HD 5870 1024 MB
4 x Seagate 750GB with X-RAID
FW - Latest
ReadyNAS Pro 6
6 x WD 1TB enterprise drives with X-RAID2 and 2-disk redunancy
FW - latest