Ok.
So by this line of reasoning you wish to argue that AEGs can effectively negate the advantage of marksman type rifles since their FPS is too low, poor shooters, and slow RoF. By this reasoning alone it would be safe to assume creating a bigger balance between AEGs and Marksmen rifles is the goal of your argument. The scenario in question is also false based on the assumption airsofters are good shots. From personal experience, the average airsofter has little experience with either firearms or real world tactics. For instance, a smart airsoft marksman would know; how fast it takes his rounds to travel, distance, personal location to target, knowledge of combatant's area, pattern predication, wind, risk, shots before interception, shooting ability, teammate position, enemy positions. and probably a few other things. That rifle doesn't make you a marksman or sniper. Because by the time you get into range of a marksman's rifle in airsoft, you should either be dead or engaged to a point you will die.
So, by your estimates about the game, why not make AEGs 300fps and Marksmen rifles 500fps?