Well Green would be our best goalkeeper if he came (he's just pulled off very smart save for West Ham as I type) and miles better than Harper. Problem is that he would get in the way of our youngsters.
I used to rate Green, but he seems the quintessential confidence player. Reminds me of Gomes regarding the incredible saves/awful clangers ratio. Personally I don't think you have an urgent need to improve on your goalkeeping and at least Krul (I haven't seen Forster play much) looks a better prospect.
tbh, I'd be against this signing because, as you say, Krul looks a good prospect and I want him to be given the chance.