I was there the same week as Pop Warner in 2004. During the day at the parks it didn't affect crowd size at all. We barely saw them during the day. The problem I had with Pop Warner was sharing buses with them (we stayed at Port Orleans FQ and there were tons of Poppers staying at Port Orleans RS) and you saw alot of them in the evenings at the parks and especially at DTD...it was MOBBED with them.
The pools were also a bit of a problem at night. We only had a couple teams staying at POFQ, but they took over the pool at night.
They also closed MGM one night early for a private Pop party, which was annoying, I wish I had known that so I could have planned another park for that day.
Basically during the day Poppers are competeting and you don't see much of them, but believe me, a bus ride with a load of them is no fun and the pools and parks in the evenings sort of get over run.