The sunsun 302 is <100 GPH: more like 66GPH. I have two of them myself.
Do you have the in and the outlet on the same side or opposite ends?
It wouls seem that should you want to contine to use your SEIO 620, I would the intake and the spray bar on the same end. and put the SEIO 620 at the other end of tank.
Hope this helps.