i was there last year mate, and i live in Germany so i know a little about the attitude of european cops. Basically, tinted visors are ok as long as it's daylight and good visability. You may get pulled for wearing them at night or if the weather turns so carry a clear one with you. Loud exhausts are not too bad, the thing being that there will be so many bikers in Holland around that area that it's very unlikely for cops to pick any one bike out of the crowd for having a noisy exhaust, but if you're riding in a small group through villages/towns away from Assen where there will be less bikes then you may attract a bit of attention, but you should be ok as hundreds of brits will be traveling over at the same time.

As you get closer to Assen there will be plenty of speed cameras, especially on the roads around the circuit near the camp sites, so be aware.

Apart from that, ride carefully, there will be loads of bikes around, and on the way out at the end of the weekend the roads will be chocka with bikes trying to get home, and the chance of the odd crash or two will be highly increased.

In summary, around Assen town and the local area, police tend to be a little leanient as far as visors / exhausts go etc. But will target guys who decide to 'invade' Assen town with an attack of wheelies and burnouts. And with busy roads, be extra careful.

Oh, and in the campsites at night, lots of drunken idiots (ok,me included) tend to enjoy reving their bikes and doing burnouts on the grass with their exhausts pointing into their mates tent while he's sleeping. The campsite wardens will just call the cops who will confiscate your keys untill the next day.

Hope that helps, BTW, i can't go this year cause i'm away so i'm gutted cause it was a fantastic weekend.

Have fun dude!