I rode an old-style pre-unit 500 years ago - a friend's wife bought him one as a birthday present and reentry bike, he's since moved on to big BMW ADVs - and if memory serves me well enough I'd never trade my J model 350 for the very marginal increase in "performance" that bike had. Granted, it was a carburetor equipped 500 Bullet, still fairly new so not thoroughly broken in, but it didn't have anything like the effortless, smooth, usable power of my current Classic. From what I've read the UCE motor was a noticeable improvement over the pre-unit version. The 500 existed, I would guess, because it had always existed, at least since the mid-1950s, in RE's catalog and the Madras factory just kept building them.
And from articles - mostly from the great Kevin Cameron of Cycle World - it's a tough job getting air-cooled motors to hit emissions targets the larger the bore and higher the piston speed goes, I'm guessing the J model 350's life-span will be relatively short and it will eventually be replaced by some version of the liquid cooled Sherpa motor. RE specifically designed the 650 twin for the export market with known future emissions standards in mind - I wouldn't be surprised if the new 750 doesn't gain much in peak HP but rather gains torque and moves the power peak lower in the rev range. While meeting those emissions standards to boot.
Don't get me wrong, I love singles. My first bike was a Triumph 250 Trailblazer (yes I know it was really a BSA). My bought as a new leftover Yamaha SRX600 is getting a thorough going through this winter now that my Classic 350 has rekindled my love of riding. But the likelihood of me replacing either bike with a new, air-cooled bigger single is slim - our world just isn't going that direction.
... When we were in the rain, in a muddy gravel construction zone, two up on this 800 pound bike, I will agree the hairs on my neck were standing up...
