I feel like your pitching is pretty good, although Verlander would be an upgrade. I don't expect McCutchen to have much of a fall of in the second half of the season, the guy is a beast, 4/5 today with 4 runs, 1HR, 3RBI, don't get me wrong I am probably a little biased, I am a Pirates fan but this McCutchen is MVP caliber. I think its a fair trade, I don't really see anything wrong with it, but in my opinion, I don't like giving someone up that has that much production everyday (McCutchen) for someone that gives you production 1 time every 5 days. You have a lot of pitchers already, and I believe most of them are giving you pretty good numbers. I feel its a lot harder to replace everyday players, unless you are just stacked in your outfield, I probably would not make this trade. But I also don't think its a bad trade if you do decide to do it.
I love McCutchen. I also love JV. But he isn't having the kind of season I hoped he would have. If I had your team and I was made that offer I would accept without much thought. You do have good pitching. Losing JV is huge but as said above. Getting a guy like Cutch that plays everyday and is a top 10 player is a very nice return.
I'd stay put. McCutchen is carrying the Pirates towards a playoff run. For once the Pirates are looking to add talent, versus reducing talent and picking up prospects. I think that mentality will help McCutchen stay hot. Also, if you deal McCutchen, you're going to be relying on Craig, Jennings and Ellsbury staying healthy.
Its a fair deal, but I think I stay put especially since you're in first place. No need to mess around with whats working.