Tyre widths...

nv55 I run a 21mm tyre (Michelin krylion carbon) on the rear because my clearances to too tight for a 23. Can't say that I notice much of a difference between that and 23s - a bit of a harsher ride maybe, but then I run at a higher pressure than I run 23mm tyres. I haven't disappeared down any drains lately, I mean - the difference is only 2mm.

To set right a couple of wrongs: In order to hit the rims on the road in a corner, I reckon I'd have to lean the bike over until it was practically on the floor. I'd lose traction long before I hit the rim on the tarmac. Also, the amount of tyre rubber in contact with the road is purely a function of tyre pressure and weight of the rider - not tyre width (this applies to bicycle tyres, not car tyres which are flat). Newton's third law.

Correcto. Hence my earlier comment.