It could be the drivers, supposedly these new 18x.xx drivers give a good performance boost... I don't know as I never tried them on the vanilla 260's but:
Single 192 vs Single 216 both @ 675mhz the 216 comes out ahead for me by around 3-5%
SLI 192 vs SLI 216 (again, this is with the new drivers) the 216 hits 10%+
