#1 is tires, though that isn't going to help body roll.
#2 is stiffer anti-roll bars. That *may* be grounds for denying warranty claims on your roll-bar end links and roll-bar bushings, but that should be about it. Certainly it shouldn't affect your powertrain or corrosion warranties, for instance. Go google "Magnuson-Moss..."
#3 is a full STi swap, likely overkill.
I'd try a different dealer if there is another one nearby, sounds like this one isn't exactly "mod friendly." BTW, your warranty is valid at ANY subaru dealer.