Well, as for the 255s they are 10 mm wider than the 245. I can tell you the 275 on an 8" rim is 1" wider than the 245. I'm sure it would be better on a 9" rim, but it does work and it doesn't look as wrong as you might think.
I physically measured the 275/40/17 tire on a 17 x 8" wheel and it's measurements were nearly the same as the listed size for a 255/35/18 tire on most tire manufacturer's websites. The wheel and tire is still sitting in my garage. To mount a tire on a non-recommended wheel size doesn't make sense from a performance and liability point of view. It's like the hella-flush import guys who stretch a 215/50/17 wheel on a 10" wide wheel. The only people who benefit from that are those who are competitive drifters and those cars aren't street legal.
As said before, get the recommended tire size for better performance and liability. It's honestly not going to look much different.
