Belt power transfer is limited first and foremost by the amount of surface contact, and the least contact always occurs on the SMALLER of either the drive or driven sheave.
As you mount the shafts closer together, the arc of contact on the smaller sheave decreases, and the larger sheave increases. The smaller getting smaller means the contact surface of the smaller is under higher stress.
It doesn't seem like it'd be that serious, but draw a very large and very small circle, put them just a fraction of a hair apart, and draw the belt path, you'll see how dramatic the contact reduction IS.
So the answer is: Farther you get, the better the wrap, and closer you get, the less you'll be able to transmit.
Will it impact you? Pull 'em close, wrap a string, and mark the sheave... now move it back, mark it again (with different color ink), and measure the circumference of both, and compare the change.
You can BEAT this, by installing idler sheave on one side (the slack side, usually) to increasewrap, but with added stress on the belt (having to make a flex in one direction, and back the other).
If you're not asking for much belt power in the first place, it won't matter. If you're already slipping the belt, then it might... all depends on magnitude and load.