I've been following this 5/3/1 variation for a while and really enjoy it. Up to until yesterday, however, I haven't been doing the ball slams and throws. I picked up a 6 pound medicine ball yesterday before the strength workout, slammed it for 15 reps, and felt my member tingle a little, so I'll stick with them.
However, is 6 pounds enough? I'm pretty sure Wendler prescribed them for speed and explosive training and the weight didn't slow me down. I tried the same exercise with the 10 pound ball and felt like it was slowing my slams down and I couldn't be as explosive as I wanted. I can see using a 10-20 pound ball for strength training, but I don't think that's the point.