Like Pete, I use a 750 in the walkway, but also one in the trunk (he uses or used to use a Fat Seat instead). The weight in the trunk is REEEEAALLY far aft and requires you get a LOT of weight REEEAALLY far forward in the boat. With the v-drive, it's best when about 50/50 like I run, but in your X you can get by with a little less up front.
If I had an X and was looking for more weight, I'd either get a Fluid Concepts 1000-pounder for the trunk or have a custom bag made to fill her up.
I'm not sure of the construction of your d-drive back there, but in my S, sometimes I get concerned about the force against the bulkhead on the forward walls of the trunk. Eventually it'd be possible to push the composite board into the engine drive belt or do other nasty stuff. This gets worse the higher up your bag rises on the wall. I'm thinking that if you really wanted to maximize the amount of bag back there, you'd want to go custom with the sack, and you'd want to reinforce that back wall by either using a tougher bulkhead material or by running a reinforcement across the back (forward side of the forward trunk wall) supported from the frame surrounding the engine. For you saying that's overkill, think how much side pressure there will be on the bulkhead with a bag 30" tall and 25" or so inches wide at the top, 30" in the middle. (I'd never go that big myself).