Well, having (at last) found out how to ride fast enough to get close to the front bunch, I know that most of the 620's are doing similar top speeds, DEPENDING ON GEARING cos snet was great for that. The thing that made the big difference was the corner exit speed, as griff on his 583 showed me by stuffing me on the inside at sears (more than once) with so much more corner entry speed and also getting the power down so much sooner than me that even tho my bike was faster than his, he still made it to the braking point under the bridge sooner than me, and then braked less there too !
We have a number of VERY good racers in DD now, and lots of them have set up their bikes REALLY well so they can carry corner speed confidently and make clunky old air-cooled engines (even 583s !) perform to their best. Spectating must be really good now, but trust me, mixing in with them is absolutely mint.
Respect
I just wish they would all publish all their set up details