to be very honest 300 deep is very shallow for a marine tank, if this can be increased at all you will see a big difference especially once you start putting LR in there.
There is nothing wrong with your sump design per se. Some okes design the buffers so that it skims the surface of the refugium. I would recommend you add a skimmer to this chamber, even if it is a small one, I don't think your tank is big enough to be considering a refugium, I would probably consider a DSB instead.
Muz