No amount of polygons can achieve the same fidelity as would a higher resolution. You'll lose definition and inherent sharpness on any size screen especially the normal screen size people are using nowadays. If I were to be using a 19" to 24" screen, I'd be hard pressed especially after Prologue but could probably live with 720p. But with my 32" 1080p screen, I'd rather not. It would be different, had GT:HD and Prologue been produced at 720p but they weren't. Personally, the question is quite academic: PD and Kaz will keep the maximum 16 car grid and the current resolution (al least) and with what Sony's other developer, Naughty Dog and Guerilla Games has understood when it comes to utilising the SPUs in the Cell processor, it's pretty safe to assume that GT5 will further optimised the codes that made Prologue possible.
A simple pixel count (the higher the better)
1280 x 720= 921,600 pixles
1280 x 1080 (Prologue)= 1,382,400 pixels
1920 x 1080 (True HD)= 2,073,600 pixels