I have been playing a lot of +300% vs +300% handicaps lately, and this is what I do know happens.
AI:
Reinforcements spawn at least roughly 4x as often
Player:
Incomes are all 4x faster
This includes knowledge
Does not change max knowledge per planet
SUPPOSEDLY it is supposed to make waves come 4x faster at the regular wave sizes for no handicap from what I have read, but I haven't seen that happen. It would be pretty silly if it actually changed wave sizes in my opinion if +300% ai vs +300% player is supposed to be balanced roughly like +100% vs +100% but faster like Chris has said before (from what I recall). Player economy going 4x faster does nothing anywhere near an effect as powerful as the AI having 4 times as many units per attack wave because wave difficulty is almost entirely based on the player ship cap:wave size ratio, not how fast players build things. Difficulty setting already increases wave sizes too so it would be redundant unless handicap also increased player ship cap (but you can just select high ship cap for that effect already as well if you want both sides to have more ships). If by design it is supposed to increase wave sizes, it should really just do wave frequency instead to have the AI get the same kind of bonus or penalty as the player does from handicap. They player doesn't get to attack with 4x as many units at once with +300%, so the AI shouldn't either. Alternately, have handicap boost player ship caps too, but CPU limitations become more of an issue. Increased wave frequency instead of size when using handicaps is probably better for CPU reasons too.
I set AI wave size to 2x frequency to at least achieve some of what it is supposedly supposed to do, but I'm not sure that's working right EITHER. The randomizer loves handing me turtles, ninnys and fortress barons though, so it could just be the recent run of all defensive AIs making it seem like they aren't attacking 2x/5x/8x as often.
