If you overflow and return "antisiphon" is setup correctly you shouldn't lose much more than an "inch" in powerout...the extra 1/2" volume is a "safety factor" ...so the 4" in the sump should be adaquate...
14" is a good depth for the fuge...
I'd widen the baffles between the Overflow/skimmer chamber and the return pump chamber to 2" or so (return pump chamber 9" ) and you should be good to go...