Chronograph testing shows that reducing the 308 barrel from 22" to 18" will give you 150 fps loss. This is an average value dependent on the actual gunpowder being used. As suggested by RL556, this will be unnoticed when shooting at 100 yards and really unless you are shooting 300 yards and beyond the downrange energy and trajectory is not that much lower. On the other hand, if you are looking for very best trajectory and utmost accuracy then you either need a 24" barreled bolt-action or a highly worked over AR-10 / SR-25 or any of the high-dollar accurized M1A's to the tune of $2,500. You can spend $2,500 on a bolt-action 308 and really have a shooter (I know!)
Just like it almost allways boils down to, What do you want to do with it? The shorter barrels are handy, but will give up a bit of velocity, just being short is in no way bad for accuracy though. My personal pick in a 308 semi-auto is a FAL with a 21" barrel. I chose 21" not for balistic performance but because I like the balance and appearance of this standard configuration.
One more thing, If you keep the M1A iron-sighted (or FAL or G3 for that matter) how will you know if it is capable of better than 2 MOA? Personally this is all the better I can see/shoot, so my NM Garrand and FAL are both in the same leager as far as accuracy with me behind them. Only with a scope fitted can I personally appreciate any more accuracy.
Of course, Your Mileage May Vary.