It's not a whole lot of extra work to convert to an 8-Tenax. Most of the replacement tops don't have the hardware installed anyway; they come with 6 loose Tenaxes, just re-use 2 of your Tenaxes from your old top when you're installing 'em. I didn't make my new top into an 8, but I thought about it -- turns out fresh Velcro makes a world of difference.
(My top was from LBC Car Co. online, just 'cuz they were on sale at the time.)
I think the back window sitting a bit slack is to keep the strain off the zipper. My top is tight, and the stress is carried around the window, but not to the window itself. It's not floppy, but not drum-tight like the old one was either. If it was <span style="font-style: italic">too </span>tight, working the zip would be a chore; and once the zipper pops, it's done. I'd rather have it fit nice and easy through there.