Most store bought HOT Overflow boxes do not allow as much flow as they are rated...
A 1,300 gph return pump will not actually return 1,300 gph. There will be head pressure, friction loss from pipe and fittings, etc... Plus different brand pumps will have different performaces under head pressure...
Without knowing all the details of your plumbing and the manufacturer of your pump we couldn't even begin to assume what your actual flow rate will be...
But, your average quality 1300 gph pump will push more than 300 gph with your average head pressure & plumbing resistence... plus your average 300 gph overflow won't allow 300 gph to flow through it...
So in conclusion, you probably have a lot more pump than you need... or not enough overflow box... depending on how you look at it...