Your drive belt will always stretch, it has too. However, when you install a new belt, that is why you have to break it in, 100+ miles, depending on sled and belt.
if you do not, there is a high chance that it will break, you wont get the right RPMs and speed. that you are looking for.
When you replace a belt, it is usually due to high wear, cracking, splitting, threads showing or severely burnt on the slides. Check and add or take away belt deflection depending on how tight the belt is. Most sleds have a spec., some guys says until it doesn't squeal. Check your manual if you have one or check with a dealer.
If the belt is too tight, you wont get any top end, but will be snappy on acceleration only for so long. If too loose, the belt might sink into the secondary, no acceleration or little and more top end, belt might twist or break.