I didn't downvote, but here's what I think:
It's not quite clear whether you want every additional 1K to add a day to the bounty. If you do, then very high-rep users will be able to place exceptionally long bounties - 365K for one that lasts a year. If you don't (i.e. you want any user over 8,000 to have exactly 8-day-long bounties), that seems more like glam than privilege, and not very visible glam. Every existing privilege does something visible (see the 1K avatar mouseover expansion), powerful (see the 500 review queue access), or interesting (see the 10K tools). I don't think anybody except the 8K-er will notice a one-day longer bounty, and if they do, there will probably be confused bug posts about it.
Back to the "stacking" case: I have a serious concern that such a system would effectively make rep have way more buying power for answers. Currently, people who are really intent on getting an answer can just place another bounty if one expires with no good answers. Allowing super high-rep users to put up long bounties with no such recurring price doesn't sit well with me. (But maybe that's just because I'm nowhere close to those numbers.)