Matt, sometimes you can advise till youre blue in the face,but i think youve done all you can and more,and id be surprised if your advice is taken on this occasion but i hope for safety sake it is.
Adam,have them powdercoated as its very hard wearing and the paint finish with aerosol cans wont be as good,its harder to do than i thought when i tried it,you can exchange your wheels for ones done so you wont have to wait.
Stuart,is your car a manual or auto as i have a manual and thought i did well last week with 29 mpg of urban driving.If you drive an auto thats very impressive mate.
Gee,that 200 quid might be the difference between an is200 and 300,id save it.Also carbon wrap is difficult ,i have tried and failed,but maybe im just rubbish at it.
From what i hear theres not much difference in mpg between the 200 and 300 in auto versions,and i think the altezza is better on fuel as its only 4 cylinders.
Gee,if its power then im afraid unless its forced induction(turbo or supercharger] then NO.Otherwise try an induction kit or panel filter or an aftermarket exhaust,but we are talking minimal or no gains.If its power then swap car for an is300 or altezza rs200.
My daughter had similar with her celica vvti,and had to have the heater matrix changed.good luck mate with the fault and welcome to the forum,there are many experts on here to help so dont despair.