Improving Skype quality
I think the only way to boost skype is to do l7 filtering, as you said. However, the skype L7 pattern is overmatching, because skype is really hard to identify. On the BlackHat Europe, March 2006, there was a very nice presentation about Skype, its security and how it obfuscates its traffic.
The only thing i can think of that it tries to generally detect interactive connections and boost those. "Skype optimised" sounds better to the marketing guys than "QoS" ;)