The change to my car was astounding, especially when combined with being lowered. While I might not run [bolt on] spacers on track, I'd say a high quality set from a good known source with quality materials is fine for the street. Of course, if you're squeamish about bolt on, even a set of 5mm slip on makes a decent difference and simply uses your existing hardware.
This is a terrific source:
http://www.motorsport-tech.com/
The new wheels are effectively the same offset as if I was running like 20-22mm spacers so they should give is the wider stance, but keeps the tires neatly tucked under the car. My rears are a _touch_ too much, but I didn't want to trim the lugs (at least the time), in case I went with new wheels later ... and that moment has arrived