The actual flow to the sump will not effect skimming much, whether you have a sump turn over rate of 200 gph or 1,000 gph, your skimmer will not be able to 100% clean the water going to it. In some ways, lower flow through the skimmer is better... i.e. recirculating skimmers. The idea off feeding the skimmer off the overflow is that only tank water and not a mix of tank/just skimmed water will be in the skimmer. This further negates the effect of sump turn over rate... At least this is my experience.