As others have said, the trunk is not adjustable... unless you want to get involved with cutting and welding. And if you do, you want to be careful about shifting mass rearward, in terms of handling; make sure you pack only light stuff in or on top of the trunk.
From my reading assorted bike magazines, I don't think the GW's trunk is adjustable, either. However, one (or maybe several) of the newer H-D baggers comes stock with an adjustable trunk; as I recall, you can move the TourPak fore/aft two inches. No such luck on the XCT.
(BTW, re one of the responses, the XCT comes stock with the adjustable floorboards. This is one of the features that influenced my buying that bike.)
My wife's preference was just the opposite, regarding the backrest angle. On my Valkyrie Interstate, I bought an aftermarket passenger backrest (from Ultimate Seats -- the backrest only, not the seat(s)) so that the angle for her back was more up-and-down, i.e., not leaning back as much. On my 650 Burgman, I had a custom-made backrest (made by a local upholsterer). On the XCT, she pronounced the stock backrest pad fine.
My recommendation for more passenger room is that you remove your backrest when traveling two-up, and put it back on when riding solo. Is that an acceptable option?
2012 XCT (SOLD)
: HeliBars; Centramatics; PC-V; Lloydz air filter; 11" Madstad; Rivco LED mirrors; Doran TPMS; Stebel horn; Motolights; CustomLED trunk flasher; KewlHeel shifter; CycleOps trunk rack; BeadRider seat; Vibranator bar ends; Passenger grab handles; Powerlet outlets; Handlebar RAM mount; RicZ laydown plate; Brukus SaddlebagSecure; 14" flexible antenna; RAM cupholder on trunk; shock Schrader valve relocation.