Things that make me go huh ?
 

Recently I switched my 18`s BFG T/A TWR for the winter set 17 Blizzaks. I know the BFG`s are noisy but I do like the grip. But when I switch to the 17 even being a snow tire the ride is different. I know contact patch etc tread but the car itself feels different. Engine does`nt work as hard. I don`t track anymore not that I would turn down some free lapping .My guess is the geometry of the car was designed for 17`s. I need new summers and I`m seriously thinking of going 17`s. Anyone else done or considered this ?

More air in tires and more sidewall results in smoother more compliant ride. Greater slip before tire breaks loose. Lots of good things with 17s. I have a 3.6 monster and I love my 17s.

Generally speaking, the 17" wheel / tyre combination is lighter weight than the average 18" combo (unless you go forged, high cost wheels). That lighter weight really contributes to better acceleration, more accurate turn in & better mid speed response from the engine as its turning less reciprocating weight. And the suspension is not working as hard too, responding better due to less mass.
A little like comparing a lightweight flywheel / clutch assembly versus the standard items.

I cycled competitively for many years and the old adage was that saving an ounce on your wheels was equivalent to saving a pound on your frame. I don't think that formulae has changed.....

Yes the reduction of unsprung weight is a huge plus.