Still yet to find the perfect 172/182 (im tres fussy about second hand cars!) so i cant comment on its performance. I can however say that in my Seat LCR, in the heat with its turbo it would be what I would call verging on sluggish. I remember joining a dual carriage way and a 1.2 16v Clio 4 down came past music booming, i booted it, more to get away from the damn bass than anything else. I was shocked it was only a few car lengths behind :O Was so shocked I immediately popped the bonnet at home to find a problem...........everything was perfect
I guess in short, heat can play havoc with performance and everyone car has its "bad days". Get out there on a cooler day with no fuel or passengers and Im sure you'll love it again!
RE petrol, Ive only ever noticed a better idle when Ive used premium, never performance gains.