The Cubs farm is mostly garbage so I can't imagine not being underwhelmed by the return if they acquire Happ. Even then I'd say that's a very ambitious suggestion by the Cubs guys. No one from the top 10 of a farm system like that? Obviously kidding themselves.
I'd say the Cubs lack both a high end quality prospect, and also the depth of quality required to beat out other teams for Happ.
Perhaps if they packaged together of one their two arms from last year's first round, plus an interesting lottery ticket (Lange/Little + Amaya) maybe that beats out the Yankees/Brewers etc, but those teams seem to have way more to offer.