I have followed Nobby on his 600 Comet, up hill, on my Shadow, in Scotland. Even with a 7 foot Dutchman and his 6 foot (but slinky...) lady, Ate, on the back, it went astonishingly quickly. I was really very surprised, since a one-up twin ought to catch a two-up single in a flash. I could catch him, but it wasn't quite the push-over I'd expected. This seems to be a near universal opinion: Comets are nice but fairly dull, 600 or 650 Comets are just as nice, no less reliable, and far from dull. I imagine it would be the perfect vehicle to terrorise Manhattan as the noise reverberates down the canyons. Or avenues, as you call them.
The Grey Flash was a moderately tuned Comet, but they WERE reliable. Three entered in the TT, three finished. I saw one in IoM in 1967, which had never been raced. It was leaning against the wall of the Sulby Hotel. (This was before drunk-driving became seriously illegal: then it was just mildly disapproved of.) The owner, a Cornishman, had always used it as a road bike. It was quite lovely: I've often wondered who he was, and what became of it. Apart from having no lights, which he didn't care about, he said it was a perfectly practical road bike.
A friend has taken a different approach to improving his Comet's performance: he's fitting a twin motor, because it goes straight in.
Good luck.
