Anyone used 20/25mm spacers on rear???

[chip]

You would expect wider tyres on a rwd car though surely?

Yes of course, but it was just illustrating that a variation in tyre size isnt a reason for the car to be illegal.

As Robi mentions though, it might be just on the basis of deviation from type approved values, but in the UK that isnt something the police care much about yet (starting to though!)

How much benefit would their be to fitting 205 tyres over 195?

You will have to try it with your preferred brand of tyre and see how it feels to you, there really arent a lot of generalisations you can make with tyre widths really.

Bear in mind that wider means less pressure for each inch of width as the weight is spread over a larger footprint though, so if you have a high grip tyre this is probably of benefit and on a low grip tyre probably a hinderance.
during tight cornering a wider tyre will lose grip as a result of the inside and outside moving in different size circles.

Its one of these bell curves though for any given car and tyre compound, so it depends if you are just to the left or just to the right of optimum if going wider helps or hinders for any given corner.
You'll find typically that in a very tight corner you want narrower tyres, as the effects of the different circles the inner and outer edges of the tyre are moving in are more pronounced and it makes direction changes easier (tight chicane for example) but on a looser bend you will want a wider tyre.

Personally I havent been able to really feel a big difference when going from 195 to 205 and back on my clio, so maybe the best width on a clio is 200, lol, but on the other hand (as you'll probably know as I think I took you out in my car once) I tend to drive past the limit of traction quite often with the car sliding about, and the negative side of going wider is less noticeable when you are breaking traction anyway with a fairly aggressive driving style, it would be more noticeable to someone who likes to drive purely within the grip limits of the vehicle.

[zmaster2k]

Cheers chip V informative!