madmarky
I'll try and answer your questions as best I can...
I have plenty of speed bumps round my way as well but haven't currently lowererd mine although I have had some Tein coilovers sat in my garage for a fair few months now!! I am running on 20's as well and at the moment, I am happy with the ride and ride height. For example, I went on a Rolling Road day yesterday and my sideskirts missed fouling on the ramps by less than 5mm. If the car was lowered there was no chance of me getting on there. Leaving the ride height as it is also means speed bumps really aren't an issue for me.
20's are a direct fit so long as you get the correct fitments. You shouldn't need to roll the arches at all. Mine are 9's at the front and 10's at the rear, can't remember the offsets off the top of my head so will have to confirm those when I can.
For the front number plate, I just made up a couple brackets and fabricated my own hanger in the front bumper. I'll add some pics below so you can see what I mean.
As for the exhausts, I have the full Nismo exhaust and the cutouts are perfect for it. The Nismo has the same size pipes as standard though and retains the original mounting brackets so you shouldn't have any problem with that and tbh it seems to be the favoured option to go for. I wouldn't like to comment for definate on any other exhaust but I would have thought that if you got the exhaust on first, any decent bodyshop should be able to manipulate the fibreglass without too many dramas to make a decent fit.
Hope this helps, feel free to ask any others if I can help
Some general pics of the VSIII kit & location of front plate
Wheel Clearance
Nismo Exhaust Clearance