Cheers Richard, saves me wondering and/or having to 'experiment' too !
I have used my slim (235) spare a few times (due to slow punchures via beading leaks) and could not feel much difference compared to the 255s
... but did not 'push it' too far though either
Had lots of discussions with tyre folks over the years about fitting 'oversized' wheels because 'they look good'....
and whereby "Low Profile" equates to 'and you will feel every bump on the road' too !!