Water-cooled 650s and aircooled 500s are not a direct comparison.
A properly built Bullet 612 can cruise at 70mph with no problems, and break "the ton" too, with the right gearing. I know somebody who's cruised all day at 70mph+ , along with his son on a Triumph 750 Bonneville(Meriden Triumph), and kept with him in every way, on his 612 Bullet. And that same 612 Bullet was clocked at 110mph by GPS timing, with him laying down on the tank.
Here's why:
The 612 increases both the bore and the stroke of the Bullet, while adding about 22% more displacement.
This provides significantly more torque, including more torque down lower in the rpm range.
You can gear it up higher to 19T or 20T, and still not be slow at accelerating off from a stoplight. It produces more power lower down in the rpms, so you can easily do 70mph at a much lower rpm without straining the engine.
It also produces more power up high, to a certain limit of about 5500rpm, so that you can pull a higher top speed too.
Make sure it has good bearings, an oil cooler, an alloy jug, and free-flowed intake and exhaust, and you have the equal of any Brit 650 twin. Maybe not quite equal in outright power, but very close, and the Bullet is 75 pounds lighter. So acceleration is going to be comparable.
It's not a cheap conversion, but it's available, and it can make that Bullet move up into the performance category of a Brit 650cc twin. Or even some Brit 750cc twins.
It puts some real muscle into a Bullet.
To really do it right, it would mean putting about $5k into your engine mods and associated other parts needed.
But if you bought a $2k Bullet in need of some TLC, and threw the $5k at it, you'd still be only at $7k. Allow probably another $1k for "unforeseens", that might turn up to be needed, like a racing clutch.