Just because you can run 5 matches doesn't mean you need to.
Take a page out of Warmahordes, short fast dynamic events.
40K doesn't need to be a marathon, low points negates the need for an endurance game.
You can have just as much fun, if not more, playing for 5 hours as you can for 7.
Also, keep the 1 HQ, 2 Troops + Whatever you can shoe horn into 500 points. It'll cut down on unbalanced.
-Gundam John