Whoa, they sure look nice and ... able to perform. I haven't had any slips with my heidenaus on tarmac either dry or wet, but on gravel they are not really up to the task.
As does the manufacturer say about the K66 pattern of my heidenaus, they are still meant for hard surfaces and only slightly able to cope with gravel etc.

I'm thinking of getting such Heidenaus next with a new 21'' front rim to accompany the different front tire aswell: LINK

And BTW, how does the 250 engine perofm of your bike and what is it exactly :P