I seem to be getting a much lower filter PSI than 20 PSI so if you could confirm the filter pressure it might help reconcile the difference.

Also are you sure the return eyeballs are 1.5" or just the pipe feeding the eyeballs? 1" eyeballs are more common and generally use 1.5" pipe to feed them.

Also do you normally run with all suction lines fully open and all return lines fully open? For example with the steps and swimout jets, are you redirecting more water to those than the pool returns or is everything have equal flow?

But just given the number of suction and return pipes that you have, 1.5" is plenty. The parallel runs, if all fully on, will have about the same head loss as a single 2.5" pipe so there is not much to gain from that. However, this also means that most of your head loss in is the pad plumbing so you will be able to achieve quite a bit of gain with just changing the pad plumbing. However, I would also change the valves because they tend to have more head loss than just fittings.