People dismiss memory bandwidth because it doesn't serve any purpose in most cases. Quad-channel DDR3 1600 provides a theoretical 51,200 Mb/s bandwidth. That just isn't something that serves any real purpose on current CPUs in most cases. It won't result in a "5% or so" performance increase in most applications because it simply won't result in ANY performance increase in most applications.
Excess L3 cache similarly serves little purpose. I'd like to see a comparison between a 2500K and 2600K at the same clock speed with HT turned off. You'd see a ridiculously low difference in performance, if any. Six-core SB-E will likely make effective use of more L3 than 2500K/2600K do, but I don't see either requiring a full 15 MB to be functional.
Extra PCI-E lanes, however, may ultimately be quite useful. I've heard complaints of LGA 1155 being a mediocre workstation platform because of the lack of available PCI-E lanes. SB-E corrects that issue. It also eliminates the need for an NF200 chip, which does decrease performance slightly.