From what I've seen of Pogba I think it's fair to say that while he has done excellently in the Serie A that doesn't neccesarily mean he would instantly come in and set the world alight abroad. When you watch these highlight YouTube montage videos pretty much every time he gets the ball and everyone gives him about ten or so yards of space to shoot in. That wouldn't happen in the premier league, someone like Clattermole or McCarthy would come flying into him and I'm not sure how well he would cope with that to be honest.
He's a huge talent and in 4/5 years he may well be the best centre mid in the world. But as it goes I'd rate Modric, Iniesta, Kroos, Verrati and possibly Thiago over him in terms of proven midfield purchases.
Put it this way if we spend near 100 mil on Pogba this summer and ignore serious deficiencies the squad has then I won't be hopeful for next season.