It's probably a little late, but here goes anyway.
Two things come to mind. First, if you were concerned with the back pressure being an issue, all you'd have to do is either put enough holes and/or make the holes large enough to handle the complete amount of water the Fluval is pushing out.
Second, I don't see how limiting the output with a spraybar would be any different than using the flow control built into the canister. That would make my first suggestion mostly irrelevant, but personally I generally don't run my 305 at half capacity so I would want the full flow potential in the spraybar.