The max I have done is 600 points AotP army + 250 points of Heroscape army (which start on the board) playing according to
TREX's Heroscape/AOTP Mixed Play Rules. I think 750 is probably a max you want to do with summoning because, as you said, you may not be able to summon everything before your Planeswalker is dead, unless you just spend the first 3 turns in your start zone summoning. That is why I think mixed Heroscape/AotP is probably best if you want to do high point games. But of course, since points in each game are not equivalent, you have to have each player have relatively the same amount of each type of army (I always do you can donate up to either 25 or 50 points between AotP army, AotP spell deck, and Heroscape army).