Here's the full rundown of the changes on the invasions with some math.

This change went into effect early this afternoon, number of invasions sites active simultaneously has increased from 2 to 3. Frequency has gone up from 2 every 3 hours to 3 every hour and a half. Meaning six active in the time that 2 would've been active previously, that's significant and will be important in a second.

I tested using a level 65 Priest I was working on. XP was cut in half per stage, currently at 65 it looks like this:
Stage 1: 10150
Stage 2: 20250
Stage 3: 15200
Stage 4: 30400

After a bit of quick math, I figured out that the XP nerf to Invasions still ends up being more raw XP per hour because of the increased occurrence of invasion sites.

456,000 XP VS 304,000 XP at level 65 in a single 3 hour interval.

That was calculated with full heirlooms and no potions, considering the XP nerf I wouldn't recommend using potions at all right now for Invasions.