The Defense is perfect the way it is right now not just anyone can be effective anymore on defense as gunner you actually have to aim and you can't lame people from across the map. Long plasmabombs are only an issue really on one map that I can think of - 3level1 the rest of the time they get defused easily and are a waste of 30 seconds of playing.
The sentry is easily countered if you use a coordinated attack. Use one attacker to draw the sentry away and one attack to grab. Or for that matter just destroy it as borg. Phish destroyed my sentry on every run basically during practice where he actually made it sorta close to the flagroom. I think most borg rely too much on the plasma alone. Going off the top balcony on 0fear with a well timed kamikaze can take out a sentry pretty easily as well.
The problem is when a long plasma is laid nobody adapts to the situation on defense in a lot of cases. The defense should be repositioned to cut the offense off and disrupt the effect of the plasma.
As far as the defense being nerfed from 3.0b to 3.3 I see a difference in the mid to lower level clans in wfa. I see a lot of high scoring games there and maps where gunners relied on altfire like element have turned into relative capfests except for only the best gunners who seem to be basically uneffected. If we went back to a 3.0b cone and damage/push setup people couldn't hanle it because it would simply be too hard. 3.3 encourages creativity. I've seen D arsos, D marines, and all sorts of other things in 3.3 that I didn't see in 3.0b ever. I welcome the change. The people that I see bitching about the gunner being weak now are people with weak aim that relied on altfire in the past. Now these gunners suck because the marine gets close enough to DM and aiming isn't quit as easy since you can't pin the offense as easily.
My suggestion is how about just try learning to adapt. Try a coordinated defuse sometimes it works pretty well. A good D marine will shut a borg down on more than half of the wfa maps that people actively play right now so I dn't see what all the bitching is about. Just adapt and go on with your life.