Will you notice a difference? Depends on how finely tuned your butt-meter is, but you'll probably notice a small degredation in ride quality and acceleration with a 15-lb wheel.
The early wheels were in the 11-13lb range, iirc. I know the 15" '99+ alloys are ~13lbs each, the 15" BBS are 12lbs each, adn the 14" BBS are ~10lbs each.
Unless you're looking for that last tenth at the track, I'd say go for it. 15lb wheels aren't that heavy.