A 2" pipe has twice the cross-section of a 1-1/2" pipe. Cross-section of a 1-1/2" pipe is 1.767 square inches, while the cross-section of a 2" pipe is 3.14 square inches. For this reason, flow can be significantly better through 2" plumbing.
I recently had to replace my sand filter tank due to a seam rupture. I replaced a 31" top-mount/multiport valve (Hayward 310T) with a 31" side-mount/slide valve (Hayward 310S). The new side mount tank has 2" plumbing ports, and the slide valve may offer better throughput than the multiport valve (I don't know this for sure).
I can tell you that since the filter tank change, my flow through the filter has improved SIGNIFICANTLY. I put the old filter medium back into the tank, so it's not because of new medium (although I did clean the medium before putting it back in). The main variable that has changed is the side-mount tank with the larger plumbing. (I have 2" plumbing from the pool to the pump, but previously I had to throttle down the pump output into a 1-1/2" line from the pump into the filter. Then after the filter, the discharge was upsized to 2" again, fed through a salt cell, and back to the pool.)
I have an in-floor cleaning system (Polaris Caretaker 99), and with the new 2" side-plumbed filter the improved flow to the pop-up heads is remarkable. The pool is cleaning much more efficiently, due to the increased flow.
Previously, one of the ways I knew it was time to backwash was when the pop-up heads started to perform sluggishly. They wouldn't pop up all the way, and they wouldn't rotate. That was an indicator that the flow through the filter was reduced to the point where backwashing was necessary to restore flow.
Now, I have no flow problems. Even as the pressure in the tank increases to the point where backwashing is necessary, the flow through the system is strong enough that the performance of the pop-up heads doesn't seem to be degraded.
I was looking through these TFP threads to see if there's some other reason why side-plumbed is better than top-plumbed. Whether my performance difference is due to side vs. top, slide valve vs. multiport valve, or 2" throughput vs. 1-1/2" throughput, I cannot say for sure. Maybe it's all of the above. At any rate, I'm very pleased with the improved performance of the system with the new filter tank.
Juan