Not even stable at 720p... It is way worse than GT5 in performance. Sony really made a mistake releasing GT6 with such a performance on the old console, it would have helped PS4 even more.
Especially racers should have at least a stable frame-rate instead of variable, that's for sure.
@Ethomaz: Nice selection of paragraphs of the article... 
Here are some more:
"Resolution has increased to 1440x1080 in GT6 but superior multi-sampling anti-aliasing is swapped out for a post-process alternative that is lacking in comparison."
"Comparisons with Gran Turismo 5 reveal a sequel that has added some genuinely impressive new effects - but there is a drop in frame-rate that impacts the fluidity of the driving experience."
"As it stands, performance seems to hinge on the use of four elements: dynamic lighting, the amount of cars on-screen, utilisation of the cockpit view, and weather effects."
"Adaptive tessellation has been added to GT6, but it's a struggle to see where it is utilised. In-game we see LOD 'pop' as lower poly cars are swapped in with higher detail versions as we move to overtake."
