Shields is no slouch. I remember people giving Hardy a better chance than Shields.
Shields has 15 straight wins with wins over Henderson, Mayhem, Okami, Robbie Lawler, who are all strong middleweights. He also has a strong chin and recovery, great cardio and insane BJJ.
His wrestling isn't at Kos' level, but he is more relentless and will try 5 rounds straight to take you to the floor, he has the cardio to do it, unlike Koscheck who think's hes a K-1 kickboxer at times and only shoots on you if he's in trouble.
It is certainly possible for Shields to sub GSP, but getting the fight to the ground is another story, plus even if he gets him down GSP is a great BJJ fighter too.
GSP is the best at WW. The guy is ridiculous and will probably be talked about for a long time after he's done fighting.
I think Shields can win 2 times out of 10 if they fought, GSP is too complete, too strong and too smart.
I think GSP's biggest hole is his stand-up, and that isn't too big of a hole anyway because he's pretty damn good at it and always getting better. Plus he doesn't have to worry about this aspect during this fight anyway, Shields is the probably first guy you'd want to have a kickboxing fight with if you were a UFC welterweight.