have you considered a japspeed back box from ebay with a bung , they were the ones i was going to go for until i got mine fully fitted from infinity , and i hear that infinity dont fit backboxes for £180 with lifetime guarantee like what i got no more , which is a shame but as you say your looking around the £350-370 mark for a full system , a mate of mine whos in his 60s owns a celica tsport and he recently got a full system and a sports cat for about £600 from infinity as Mr taylor was down there also at the same time but the japspeed were only something like £150-160 and a nice exhaust note would finish your GT off nicely:)