The ride quality difference between 15" wheels and 17 or 18" wheels is significant. Short sidewalls have their pluses, but ride quality isn't one of them. I've yet to see an older muscle car with 17's or 18's that looks right.
That depends a lot on what your spring rate is. If you have a stiff suspension like mine with 1.06" T-bars, it'll actually ride better with 18's than 15's (I ran with both this summer), and my lowered 04 Colorado which also has quite stiff springs in it...I use 17's in the summer and 15's in the winter. Rides better with the 17's. Somehow on both vehicles the 15's are bouncy enough to make roads feel really harsh. If you have really soft suspension like the stock stuff, it will be like you say.
