Very good tips. Testing the traction level with the rear brake is the best you can do. Of course, modern touring tires like the Pilot Road 3 or the new Z8 M/O do help a lot. Most important is to be smooth, whatever you do, do it smooth. Smooth throttle, smooth brakes, smooth turn in and you can do a lot in the rain.
In general I do slow down a bit in the rain and am less agressive on the throttle (I have ABS, so braking isn't an issue) so I end up vaguely at the posted speed limit. Through the courners I do add a little bit extra cautiousness.
If you stay relaxed and don't push it, there's no problem with riding in the rain, take a look:
"Why not stay in disguise all the time? You know, look like everyone else."
"Because we shouldn't have to."