In accordance to the swath of posts like this, we've increased the effectiveness of AA weaponry like the AC2's and 5's. They now have a tighter cone of fire over range and make Aero quake in fear To adjust this, The Partisan Prime for example (All AC5) is less effective against armor, but more effective against air.
Nice! Personally i love the Partisan prime and to be honest good call on lowering the armour damage: even though it's a very enjoyable vehicle to play i did slightly feel that it's a mite too strong for a first-buy tank. Switch the quad autocannon to chain fire, squeeze down the trigger and the next time you have to release it is when either your magazines have run dry or you've run out of things to shoot at :p I mean the thing could tear any first-buy light 'mech to ribbons with little risk to itself.
Regarding the air/anti-air stuff have you guys considered to provide flak ammunition to AA-capable weaponry (namely AC2's and 5's)?
That's of course if you're planning to - or if it's even possible - do different ammo types for vehicle weapons. Best game reference to its use would be Planetside's Burster AA MAX suits and vehicle/turret mounted flak guns: their shells did rather puny damage against ground targets but against airborne units (aircraft and jumping BFRs) the shells had proximity triggers that bathed aircraft in a flak cloud damaging them from the "splash" of the explosions - it also looked cool and was a rewarding experience when you played as airguard.
IF it'd be possible to have different ammo for some weapon systems you'd of course need to have free tonnage and c-bills to buy the alternate ammo plus switching ammotype on the field should take at least the same time it takes to rearm a dry magazine from stored ammo bay.
Then there's the obvious chance of implementing dedicated AA 'mechs like the Jagermech and the mighty classic Rifleman - them puppies would give players like OP the best of both worlds with a 'mech that's able to effectively defend against air and go toe to toe with other 'mechs.