I ran into this last year with my PB. Went with upgrade system that really wasn't much more in price. Northstar 2spd 2hp, 2.5" plumbing, 4 returns, 2 skimmers, 2 main drains, 500lb filter. I have a spillover spa, swimout also.
18x36 vinyl and run the system on low spd, with the valves set the spa spills over and the SWCG operates well.