No matter what the file system is, you will lose space once formatted.
Yes fat 32 is 32k cluster size,
Yes ntfs can go as low as 512b cluster size
But an 8gb sd card will always give you about 7gb usable space.
My spica has an 8gb sd card, with a 1gb ext2 partition for the os and 7gb fat32 for storage.
It sounds as if you only need about 4-5gb anyway.
If you really do need more space, then just buy a bigger sd card. Or if you haven't already, revamp your site to use .png images instead of .jpg or you could just produce a .pdf catalogue to keep on your phone to show clients. I'm sure that will need less space.