It's good to be back, Mr High Side.
In theory, one could graft the naughty bits from a GT onto a Classic, but in practice it's a bit complicated and would be a lengthy process. OEM spares for the GT are in short supply due to the world wide demand for these bikes, but compounding the issue is that RE does not allow its North American dealers to maintain a stock of rare internal bits like the 535 con rods, pistons, cams, EFI module and so on. The only way we (when I worked at the dealer) could get parts like that was if they were part of a warranty claim - and we had to provide photographic evidence of the failed parts first before they would ship out replacements.
Aside from that, stuffing GT internals into a Classic motor really doesn't gain much in the end anyway, as the Classic still has the somewhat antiquated traditional suspension and frame that will inhibit enjoyment in the fast lane. Dropping in the complete 535 motor at the factory is unlikely to happen, at least not until the insane demand for the GT has leveled off and extra motors are available. We were able to coax a precious few more HP out of our customer's Classics by switching to an NGK Iridium spark plug (much easier cold starting, idling, better spark at high RPM), a K&N air filter and a D&D exhaust system or, if money was a concern, the RE "off road" low restriction, slip on mufflers. The parameters of the Keihin EFI chip were able to compensate for this without needing a reflash, which is good because RE did not make access available to the chip by its dealers. I have heard, though, that Dyno Tune has come out with a plug and play replacement for the stock EFI. Coupled with the K&N filter and D&D full stainless exhaust, this should give all the extra HP that is sufficient for the Classic frame and running gear to handle.
As for the vibration at higher speeds, keep in mind that there is no counterbalancer in this motor (which is amazing, considering it is a solid mount to the frame, too) and the natural sweet spot by design is in the 80-95kmh zone for these bikes. The GT is the bike you need if you want to see regular duty in the 110-120kmh zone, as the bike has been designed for those speeds.
It isn't WHAT you ride,
It's THAT you ride