I would not install any valve on out line .They can restrict flow leading to tank overflow ...
I if I thought I needed to I would install ball valves on returns.. It is the pump that really dictates everything.. Most never have a pump too strong to cause issue with standard size overflows .They lack understanding head pressures effect on GPH
of return pumps. But if you do have a pump to strong it needs to be throttled. You can't increase the flow of the out lines to sump and pumps should only be slowed after the pump..
Average 1 1/2 inch pvc
flows in excess of 700GPH with gravity only...2 lines should yield you 1400gph flow. The overflow will never go too fast as the pump truly controls how fast water flows into it.
I have run mag 900s on my single overflow 75g and mag 24 on my 180 dual overflow .I now run a jeboa 12,000 LPH digital on the 180 and 120.
I like digital pumps as no valves are needed .You simply turn pump down[if needed] and save electricity also...Dc
pumps also transfer less to no heat to tank. The mags add on average 4 degrees to tank above room temp..
You could be all set with a 6000...
The jeboa gyres rock also .
I won't buy another wave maker again.
WP 25 /40....