As with most things on the Wilderness Classic, lighter is better. You really don't need more than a nylong bag to cram your stuff into for a backpack. It doesn't have to be waterproof but if it's not you should line it with a garbage bag to keep your stuff semi-dry. In fact everything you want to keep dry should be in ziploc bags anyway.
Some people have used super light dry bags used for rafting and kayaking. These are handy because they might actually keep your stuff dry. Most of them are pretty heavy though and not very large. You definitely don't need anything with a frame or padding on it unless you're going to carry 30+ pounds. If you're carrying that much then you're probably not worried about speed anyway.
You should try to keep your pack weight below 22 pounds. This is pretty hard to do but it's possible if you don't eat too much and don't take anything to sleep in. Keeping pack weight to a minimum used to be easier before Alpacka rafts were required equipment. These are excellent rafts and they're lightweight relative to other floating devices but they are heavier than what racers used to carry. If you add on a spray-skirt and a life-jacket, you're adding on quite a few pounds. Minimalistic is the best choice. You can add a few ounces here and few ounces there for comfort but it's going to cost you in energy and time. The main question should be can you literally survive without the piece of equipment. If you can't, you should probably take it. If you can, consider if it will increase your speed. If it won't then don't take it.
You should look for a backpack that by itself weighs less than 16oz. Most weigh more than that but it's a good goal. Cut extra straps and material off and take anything metal off of it. You'll usually save several ounces by doing this. Weigh everything on a scale and evaluate anything that weighs more than an ounce. 85 miles into the race, you'll be glad you did.
Some racers have been able to get their pack weight below 20 pounds but this is pretty rare. Anyone who claims their pack is 18 pounds is probably not counting in the weight of a full water bottle or has a funky scale.
Don't forget to use your backpack as a half-sleeping bag and the garbage bag in it as a bivy sack. GoLite makes some good, lightweight packs but there are several other brands out there that will work well also. Look at Walmart or Costco for a super cheap (and inexpensive) pack that you can cut stuff off of. If you pare your pack down to just cheap nylon and a waist belt, you're on the right track.
Line your pack with a garbage bag and tie the top. Put everything in ziplocs inside the garbage bag. Things like fire starter you probably want to put in two ziplocs. Many racers carry smaller bags like a fanny pack somewhere on their body. You won't have time to stop and take your pack off to dig for food or water. Strap your water bottle to your chest and put a few power bars in an outside pocket. You'll dig in your pack a lot less than you think. If something is in the bottom of your pack, you'll probably never get it out just because it takes so much time to unpack.
You should carry absolute survival material on your body somewhere. When your raft flips over and your pack goes floating away, you'll be glad you have a lighter and a couple calories with you.
It may seem strange, but consider what color your pack is. If it's a bright color you can wave it to flag down the helicopter that comes to rescue you when your ankles swell up like cantalope and you think you can't hobble even one more mile. On the other hand if it's a light color then you might boost your competitors' moral when they spot you across the valley. They'll speed up to catch you whereas if they can't see you, they'll still think you're 20 miles ahead of them.