Yorkfield isn't decidedly faster, clock for clock, than Barcelona at all, in a lot of cases it's slower. It was always Barcelona's poor overclocking (and terrible power consumption scaling with clock speed) compared to Yorkfield that made it look slow.... heck even that was cured by the launch of SB750 motherboards, which have no problems getting Phenom's past 3Ghz.
Phenom 9950 Review
Deneb/Shanghai is also far, far more than just a Barcelona rehash. It's done on a completely new process (immersion lithography) and completely eradicates the problems that Barcelona suffered from; IE low cache, poor power consumption and low clock yields. If the early Shanghai benchmarks are anything to go by Deneb should sit nicely between Penryn and Nehalem in terms of performance (maybe excluding the top end QX7 chips).
But as i've said, Deneb's biggest selling point is going to be it's compatibility. It's a drop-in replacement for current AM2+ boards that supports both DDR2 and DDR3 straight out the box. If the rumours of 4Ghz+ overclocks are true Deneb can do a good job of swaying people away from a costly Nehalem system upgrade.