Never had that problem with mine. Generally you'll want to see the curve, shift into the proper gear, allowing the engine and tranny a chance to synchronize, maintain that speed thru the first half of the turn. Then, upon reaching the apex of the turn, accelerate out of the turn. If you are having to make shift changes after entering the turn but prior to the apex then either you've entered too fast, too slow or somehow misjudged to the point that your trying to make a correction at the wrong spot in the curve.  Now, being that it is a single and is an overall light weight bike will amplify the same things that would happen on a heavier, multicylinder engine. It's just that those other bikes absorb the feeling more. Errors, if that is what it is, is sometimes more forgiving on your bike compared to others and vice versa.

Come on over to our side of the pond and spend a weekend at the Dragon. I'm certain a day or two there will work out any curve judging/entry speed/shifting/engine-tranny synchronization issues.  Either that or you won't have a rideable bike anymore.

Down shifting in a curve at speed is not recommended at all. Taking power off the rear wheel and putting it back on is the cause of your shimmy. Our bike has enough torque to pull you out of the curve even at low revs in a higher gear.
That's why they beet the snot out of Hurleys at the Dragon.

It was late when I wrote this and I didn't really mean I change gear in the curve - I change well before, but I'm thinking about the turn and watching traffic and if it does this little shimmy at this point it really gets on my tits. Also notice the issue when I change down for upcoming speed humps (they're everywhere in this town!)
Will try adjusting the shocks today and if that does fix it I'll go for longer shocks.
Other good cheap recommendations for substitute shocks?

I only had questionable wheel action when I shift into a lower gear when rpms are too high.

5 speed transmission and all, its still wide ratio and the rpms have to match the road speed for the gear or excitement happens.

Are you matching RPM well enough? Is it possible that shimmy is a little bit of rear tire sliding? Dang things dont take a lot before theyre being dragged down the road instead of rolling. You know how little rear brake is required to make one start to slide a little.