The way that we have our X2 (2007 with the 350) is the stock tanks, plus an additional 450 in each rear storage locker. a 450 in the bow to act like a filler cushion. This is a pretty good set up, but we found it gets better with another 150-250 on top of the front sack to make a t. And if that still isn't quite enough another 450 on the floor pushed up against the rear fiberglass. We usually don't ride with a big crew so we don't have all that much gear or people weight. We keep about 100 lbs of balance weight, which helps, but isn't a necessity.
With this our boat has no problems getting on plane, but it is propped pretty aggressively, we can pull around 23 mph @ 3500 RPM using the full set up above.
This set up is good all the way out to 80ft at 22.5 mph.
stock (650 ish)
lockers 2 * 450
optional if needed
cabin floor 450.