987 has ran 9mm-wide 5mm HTD belts for the last 5 or so years. Our outside wheels are lifted up slightly so we have a center drop, and the center wheel is driven by the gearbox. There is a 20mm wide pulley on the center wheel axle that has two 9mm belts going to 9mm wide pulleys on the outside wheel axles. I'm rebuilding this years assembly right now so I don't have pulleys in this photo, but you can see how we keep our belts tensioned properly.
http://i.imgur.com/k3bs57U.png (attached the file for anyone who cant access imgur at work/school)
There was a thread a few days ago that was very similar, and a lot of teams seemed to avoid tensioning their belts like this. I believe one of the specific arguments against it was that you were now relying on the static friction of the bolts on the rails to hold your bearing blocks in place and keep the belts tensioned, but we've never seen them come loose (although it is one of the things on our prematch checklist). A little bit of locktite and a washer on each bolt also helps immensely.
