From what I can tell weight has a minimal effect on speed all things being equal. I used a model I have and calculated the resistance at a given force of 2 lbs or if you prefer what speed the boat would be traveling at 2 lbs of resistance. Round numbers it shows a pretty consistent difference of .11 mph per every 25 lb change. Probably not enough for most of us to notice. But if you racing that is pretty big increase I would suspect.

I have no way to measure acceleration but I will say seat of pants feeling is they do accelerate faster.

If he wants a faster boat, look for multi-chined designs. From what I am seeing you can lower the wetted surface a noticeable amount and lower the resistance quit a bit too. I was quite surprised first time I paddled mine but I suspected it was just me. Then when I paddled with a group there was quite a noticeable difference in speed.