(Sorry, I got a little carried away

-the main suggestion is at the end)
With an M-16 at 200m I can hit a stationary (man sized) target about 85% of the time, at 25m I can hit a 15 cm target 98% and 4 cm 50% of the time.
Really? At 36m I can put 4 shots into a dime, and at 200 I can hit a 6" target abut 80-90%, and a head/shoulders size one 100% At 20m I can ask "which eye?"(of course, with a M-9 pistol I can't hit a barn from the inside...but one of my Sergeants can empty the magazine into a 6" target at 25m as fast as he can squeeze the trigger) With asniper rifle I can hit a mansized target at 1km (so far) at 100%
In actual combat I am a little sloppier, I'll grant, but not too much.
Mek weapons don't suffer from many of the limitations of infantry weapons- they have a much more stable firing platform (no breathing, heartbeat, muscle strain/exertation etc), better sighting equipment, and reduced weather effects. Similarly the target is of the order of 5-10 times bigger, so doubling the effective range rather than at least multiplying by 5 seems rather miserly. Multiplying by at leat 5 would be more reasonable. Also the accuracy of each weapon is different (for example my pistol shooting rather than my M16A2) as reflected in the range increments- some weapons are inherantly more accurate at longer ranges, but the feats penalise them over, say a PA cannon, especially given the high BAB requirements for many of the feats
While you can get close enough to engage at 20m (though finding cover and concealment for a vertical tank is not going to be as easy as you suggest in normal terrain- a decent sensor suite will handle most concealment) that really reduces the point of having long range weapons- why mount a railgun when I'm trying to get close enough to hit him with a PA cannon? You are forced to either ignore the feats or fight in one "up close" style, so a "Mek sniper" is just SOL.
Perhaps an alternative might be to limit the range to 20m, unless the character is stationary and/or takes an aim action, which allows him to use it out to one range increment of the weapon?