I know what they say.....but have you actually seen any truth behind the theory that players that play less than 100% are more likely to get injured and to not perform as well?

Injured, yes.  Lesser performance, not enough data.

If a player at 100% bats .300.  Then. would it be noticeable if he hit .297 at 99%?

no, however there are some that still run their ML players out at less than 90%.  At that point I would think there would be a difference and the frequency of injury would rise dramatically.

My experience is that the drop off comes in the field.
They bat the same but their defense seems to suffer.
I used to regularly run guys out til about 92%.
I try to keep them more fresh than that starting about 4 seasons ago due to injuries.

The Help screen says players play worse during the game as fatigue decreases DURING the game. It doesn't say if this is fielding and/or pitching and hitting. I personally believe it's everything because my relievers seem to blow it mostly when they are spent. I also have good luck against a tough batter every year in playoffs because the owner doesn't rest him and he plays with 80 fatigue every post season. Grain of salt but..

Thanks guys!