My belief is that the shadows are of a lower quality to prologue because of the demand for a 60fps experience. Shadows in videogames are expensive to calculate. Let me give you an example; On World of Warcraft I can run the graphics on 'Ultra' but I get fps lag in the highly populated areas of the map (Stormwind city, Dalaran etc.) However if I turn shadows down a tad, I never get any lag at all. Here's the thing, on ultra they are smooth as butter, how we like them to be, but on good they have the nasty jagged edge just like gt5 does.
My PC is more than capable of running the latest games, but shadows still kill it. I have noticed this in many games.
Of course, if there was a way to fix the shadows, by all means they should, but I dont see it happening unless Sony suddenly announces free RAM upgrades to all PS3s! and as we know, this is a ridiculous plan.
Plus, the shadows aren't that bad really... Nor is the jagged edge particle effect when drifting/rally. It's a racing sim, look at the road, not the shadows!!!