no. it's more than you need, first of all. secondly, the overflow will probably lose siphon with the smaller pump (i'm assuming you mean mag 9.5), which after head loss is going to be somewhere around 700-800 gph
you need to have a pump that matches your flow rate on your HOB
. otherwise after a while, you'll see a little air bubble in the J tube, then a larger bubble...then one day the overflow loses siphon (and the pump is still running), and that spells a wet floor and unhappy fishies.
i would use a 600gph overflow and a 700-800gph pump and choke the pump back until it's perfect for the overflow.