2 ply vs 6 ply
You are bound to feel a little difference in acceleration, you are adding 20 lbs of weight the engine has to spin to get moving, which translates a little differently than 20 pounds on the seat or racks. I would still go for it though. 6 ply tires are much more durable and the performance decrease will be pretty small. It isn't like you are drag racing the thing and need to shave every possible ounce.