Yes, extra cooling sure wouldn't hurt in any case. Especially if it's so easy to do
An O-ring is stretchable by nature and I don't know of any 'scientific' recommendations for its tension. And here is its advantage, I think, you don't need any tensioning mechanism. I designed my drives so that the O-ring is a bit stretched - about 5mm shorter than required (if memory serves me well) worked fine for me. But it would depend on the overall length of the belt. Mine were made from long stock O-ring, cut to size and melt joined the ends.
Correction: V-belts work better at high speeds. Much better in most cases! Timing belts produce noise (and heat) as the speed increases - this is a fact! As the speed increases you have to consider for precise aligning of the timing belt. As the speed increases the teeth wear becomes more of an issue. For high speed application not requiring positive belt grip don't use timing belts. There are big industrial lathes with spindle speeds to 18,000RPM capable of rigid tapping which use poly V belts - NOT timing belts.