Since you commute and live in an area where it often rains, you need to overcome your apprehension. First, get a good rain suit and waterproof gloves, and either waterproof boots or rubber overboots. It's a lot easier to deal with rain when you're
1. Just how easily do bikes hydroplane?
In my experience, less easily than cars. I don't think I've ever hydroplaned a motorcycle. Relatively small contact patches that limit our dry grip actually work to our advantage in the wet.
2. How much loss of traction might I expect on street tires in really wet weather?
On a clean but wet surface, with tires made to handle wet conditions (some sport tires aren't), you're not going to use up available grip at normal street riding speeds. But, as oldgixer noted, a dirty and
wet surface offers far less grip than one that is merely wet.
My only rain crash occurred in Turn 6 at Laguna Seca about 10 years ago. The big bikes that had kicked my Hawk GT's ass all day found the tables turned when the rain started falling. I, of course, was lovin it. There's nothing quite as satisfying as passing a much faster bike with skill as your only advantage. However, as the deluge continued, a stream of mud began flowing across the bowl-like apex of 6. And that was my downfall (no pun intended...or, maybe it was). I eventually lost the front end and slid all the way to the turn's exit. Fortunately, I rode off unscathed after bending the shift lever back into place.
3. Any other tricks to keeping the visor clear in rain or road spray other than quick wipes with your hand?
I try to keep my visor down and tightly closed to keep water off the inside surface. Look for gloves with a built-in squeegee on the left index finger and wipe when necessary.
4. Any other advice you can give besides slow down?
Stay loose on the bars and be smooth in all your actions--throttle, clutch, steering, and brakes.
And, practice. When the rainy season begins, look for an opportunity to take a nice long ride. Just put on your rain gear and go out and do it. It's the perfect time to brush up your wet-weather skills, and to make sure your rain gear doesn't have any holes and re-accustom yourself to wearing it.