I've seen Forster play behind a crap back four and he hasn't been too impressive. Saints defense this season is why Forster looks good.
I would not say he is a downgrade on Boruc but i've also never seen a display from him again like that night for celts against Barca. If he had been anything but English there wouldn't have been such hype about him, he's dave watsons pet project who wanted him out of celts for a while into the epl