Indeed, a 19" wheel will generally have fewer options and 1/2" increments are that much more difficult.
You will find the widest variety in 17"s & 18"s and a bit less in 20"s for your ride. You will not see soo much a drop off in performance going a bit narrower in tire, but more of an issue of how the tire fits the wheel. A 255 is a bit on the narrower side for a 9.5' wheel, so would expect the front to be a bit "darty". In theory, you would just have a bit more understeer going smaller 1/2" smaller up front, but I wouldn't worry about it.
You will get some variation in off-set, but is mostly between different widths more than anything else. 10mm is about 3/8" and is not going to be an issue up to a 9" wheel. If you get into 9.5"s & 10"s, you will need to be more specific to avoid rubbing.
Head over to
Tire Rack OEWheels and
American Muscle and look at the off-set /back-space between the same width wheels on each site. You will not see much variation in the 9"s & 10"s, but a 10mm is not unusual in the 7"~8" widths.
Jazzer
