I've lowered a Busa or two... About the most streetable set up you can pull off without scraping everywhere is lower the front an inch, and use the 3rd hole in the rear lowering link. (2.5in?) it tucks the front wheel nice, and the plastics sit flush with the ground.
I set up this bike on the second hole because he rides with his ole lady a lot. Either way if your a heavy hitter you'll need to tighten up your spring on the rear.
You can go lower and longer however you run the risk of smashing your oil pan. If your lucky you'll just crack it. Worse case you crack it, and put oil down in front of the tire. Not good. Go too low you could put a solid strut in the rear, however it rides like crap.
This is my personal bike. Lowered 3in in the front and 5 in the rear. Your best bet if you wanna go low is to put a low profile pan on, and trim the bottoms of your farings. My forks are cut 2 inches, and slid up in the trees 1 inch. Rear has soupy links. I still have 5-6in of oil pan clearence. I have the forks set up so they will hit the travel limiters bedore hitting the pan. even on the limiters i still have 1-2in clearence. Rides great.
Here's another bike I did. Forks cut 3in and slid up 1 inch. Soupy adjustable rear links. Again rides great, and will hit travel limiters before hitting the pan. However with the forks cut that much you hit the limiters frequently anything over an average bump...and that hits hard sometimes. So I wouldn't suggest this low if you live in northern states since roads tend to be a bit more rough.
On the left.
Other notes worth mentioning are depending how much you trim the bottoms of the fairings you can give the effect that its really slammed, or trim it higher and it will look not as low. It's all your taste, and what you can get away with on your roads.