Even though, as many have already mentioned, 8mm dia on a 1000mm span is rather small, you can still get away with it as long as you go slow (-er than your critical speed).
According to this chart (source: Zap Automation) your safe max span is 700mm.
According to this chart (source: Thomson BSA) you can reach 500rpms with B style supports @1000mm. In case this is a 2mm lead , 4-start, your theoretical max feed is 4000mm/min (157IPM).
Another risk you are facing at small dia - large length is runout (or even worse bent screws). In practice your max speed will be lower, depending on the quality (straightness) of each piece. This reinforces @BlueLocktite’s point about sticking to the plates.
This is fine as long as you (and your customers) are aware of this and accept this loss for the gain in rigidity. Between the belt and the leadscrew, one system’s advantage is the other’s disadvantage.
EDIT: With a low dia/length ratio you accentuate this disadvantage, with a high one you eliminate it.
My personal comment on the commercial side would be that I’d expect your product to be priced below a C-Beam kit (which includes a 8mm screw plus a guided rail).