Okay, here's my .02 worth.
First, I've never ridden a Wing, so I can't speak for that. However, my IMPRESSION is that for very long, cross country touring, there probably isn't anything better than a wing.
And while, yes, we've all seen those videos of cops putting wings through the cones courses showing that, in the proper hands, they really can handle well, I can't imagine they'd handle anything like the FJR.
Your main question seems to focus on the supposed top-heaviness and the potential for droppage. I, personally, don't have that issue. I'm an average sized guy, 180 lbs, 5'11'. Mine is a Gen 1, 2003, and I can EASILY stand flat-footed on the ground while stopped. In very slow turns it takes a bit of acclimating because after all, this is still a 620+ lb machine. But man, get 'er rolling and I feel not a lot different than when I'm riding my wife's GSX650F. Seriously. But I can't imagine that the FJR is any harder to hold up at slow speeds than a wing.
If at all possible, try to get hold of an FJR for a few days and try it for yourself. Borrow or rent one if possible, or, heck, if finances allow, just buy a used one off Craig's list or cycletrader.com and if you don't like it, sell it.
As far as cross-country; I've done that - 1800 miles from Atlanta to Colorado in 3 days and for me, it was rough. Great wind protection - I have the Yamaha touring screen on, but my bum was pretty sore after awhile. I was averaging 600+ mile days, but it was too much. I have no doubt a wing would have been MUCH more comfortable.
Hope this helps somewhat.
* edited to add this *: Just re-read and saw where you're stating 300-400 mile days are going to be the norm/max for you now. I'm in exactly the same boat. In that case I wholeheartedly recommend the FJR and respectfully disagree with the response below me who says "Keep the Wing". I'd only keep it if I knew cross country trips and 500+ mile days were going to be in the future for me.
(and the spousal unit rides a Ninja 500!)
Last edited by WhatYes; 02-22-2016 at 02:01 PM.