Honestly no the D220's are'nt that bad, they work adequately in most types of road conditions. They downfall is that they dont heat very quickly and they also loose their heat quickly to.
I spent 4 weeks on them a couple of winters ago on a loan bike, I was regularly doing 60 mile trips in freezing conditions, they were fine I never had any trouble with slipping and sliding, it was difficult to feel what the tyres were doing though they were very vague.
I found them OK in the wet as well, but maybe thats just me.
The thing is that whilst they are OK, the Z6's are much much superior, they work much better in the wet and where they scored big for me was that they give much much more feedback so you can feel what the bike is doing with a lot more certainty.
As for kneedown well I cant do it, but I suspect that any modern tyre, D220 or otherwise offers sufficent grip for kneedown tomfoolery
Remember its not about how much grip or sticky tyres or whatever. The main thing to get right for a successful kneedown is your body position on the bike, thats it thats all there is to it. The main thing thats stopping you is that the D220's have robbed you of your confidence, just take it easy get the Z6's scrubbed in get a feel for how the bike rides and you'll find its much more confidence inspiring. Which will allow you to lean the bike more, hang off more experiement with your body position and you'll have done it before you realise it.
There was an article in one of the bike mags a while back about some road test rider getting his knee down on the road on a winter morning on an MV with standard road tyres on, he said that any modern tyre would allow this.