The passenger seat looks to be pushed all the way back, which isn't really necessary unless you're about 7ft tall! So there is a bit more room there, albeit not much more as you say.
However, I can sit reasonably comfortably in the back of my wagon (6' 3", 17 stones), although I probably wouldn't want to do a longish journey sitting back there.
Modding? Virtually impossible to resist. The big benefit of the later Subaru's is that you can remap the existing ECU to run on Super unleaded/Optimax and make the most of the bits the car has.
In any event, if I was purchasing that car the first thing I'd would do is either remove the full decat or have the car remapped properly for it. Classic STi's can and do blow up when decatted and the map isn't altered to suit - especially if it's only been run on Optimax as the ad suggests. STi's are more highly strung than the WRX and do need to be run on Optimax and octane booster, preferably NF.
'tis a cracking car, though.