I’ve seen this talk of leaving belts loose umpteen times on the forum.
A potential waste of good belts if slippage is needed, gives no gain if fitting an over-powered motor (if the belts slip before that extra power can be used, anyway), extra heat generated if belts are slipping on every motor start (mainly single phase motors for hobbyists).
You will not find any belt manufacturer/supplier suggesting their belts should not be tensioned correctly.
That adds up to I don’t recommend that practice and it is a waste of time and expense. Fit the correct motor such that it will not damage or cause early wear to the machine – and it is much safer for the sensible users, who do fit the proper power supply to their machines.
25% would be about the maximum increase over the OEM supplied power unit I would consider. I want to look after my machinery. Doubling the power likely means it needs twice the belt rating – may need twin-sheave pulleys, for instance. Geared machines will likely require stronger gear trains (larger teeth and/or wider gears.
I doubt that many, who just over-power their machines, think of all the considerations sensibly before making the change.
Edited By not done it yet on 28/04/2022 21:51:40