For the gunwale comment:

I've seen ultra-light boats without enough thwarts to handle any type
of wave. I typically add one or two thwarts to my whitewater boats, to
stiffen the sides and reinforce them. I do that mostly on Royalex but
would probably do the same on composite. If the gunwales are not
properly supported, they will flex to no end and pop goes the weasel.

For the weight comment:

Yep, weight makes a big difference. If you are seriously looking for
performance then go Kevlar, drop the weight, and accept the fact you
may have to buy a new boat sooner than expected. I notice even a 5lb
difference in hulls, and dropping the weight even more can make a
difference. That said, losing 5lbs of fat from my belly does almost
the same thing. It's inertia, and the lighter the boat (lighter total
load) the faster it will respond. You can take two identicle hulls,
one out of graphite, the other heavy fiberglass and with the 30lb
difference it is night and day. The heavier boat does not react nearly
as fast as a light boat. You can definitly feel it. The boats may sit
almost as low in the water, that's not the issue. The issue is
acceleration and fast reaction. Heavier boats react much slower than
light boats.

Still, for someone just starting in whitewater, go Royalex. You can
buy the graphite Outrage later when you have the skills to give it
proper respect. The learning curve can be hard on boats, so buying the
cheapest, most durable makes sense. If you develop the skills not to
trash a heavy boat, you will have more than enough skill to keep a
light boat in shape.