"...while completely adequate and usable my 4.90 ounce SUL pack was a little bit of a challenge to repack in the field. The spinnaker and silnylon shell simply doesn't have much "body" when empty. Also the material is prone to siding down as I'm trying to pack up."
I've moved away from spinnaker and silnylon shells for my packs for the reasons stated above.
XPAC is a approximately 400 Denier textured nylon with a tough waterproof laminate. The nominal fabric weight is 4.9-5.5 oz per sq yard dependent upon which "color" you choose. The grey seems to be the heavier fabric in my experience.
I used 500D Cordura on the bottom of my most recent pack for reasons of durability. In retrospect I'm not sure that the Cordura is anymore durable than the grey XPAC. Chris Zimmer seems to have used the grey XPAC material on the bottoms of all his packs.
"My requirements are the standard three: waterproof, durable and light."
We have covered waterproof and durable. How large and how light do you want your new pack? You could combine the XPAC with cuben and get light, durable and waterproof. Your wallet will be lighter also! ;-)
Take a look at Chris Zimmer's 10.5 oz Cuben Fiber / Xpac Backpack.
There are many options to consider. How light do you want your pack and your wallet to be when the new pack is complete?