Here's the thing. The damage proficiency changing actually helps out AV a lot. Here's the current situation.

1. Armor tanks can get regen in excess of 3x the regen of shields. They can rep 4000 armor in about 15 seconds, meaning they just have to circle around and be back at full health.

2. Unlike shields, you can't stop the regen of armor. that means that even if you are hitting armor, they are still regening that armor back.

3. The new proficiencies will improve your armor damage by 25% (word is that the new proficiency will be 5% per level) which means you will now wreck armor like no ones business. They have the most hp, and you'll be able to chew through that HP much easier now. Shields will be an issue for swarms, but forges can still shut down their regen. You can't do that with armor.

All in all, AV took a hit with the damage mod nerf, but it won't affect you very much against armor vehicles, and may even make them easier to kill than they are now with the raw damage buff of 3%

Could you please point out to where you got your information that proficiency will be 5% per level now?

I find that hard to believe as that would just be counter to all of the longer TTK changes coming in wouldn't it?
It makes no sense to reduce damage, only to give it back by increasing the prof. dmg % per level.

Even still, a forge gun for instance, is still going to lose damage both from the DMG mod changes and also will lose out when firing at shield tanks.