the only problem i see is still getting your hands into the sump if you need to. for example, to clean or remove thing. 3" seems like a good gap (its exaclty the same gap i had on my old tank), but when you have to reach in and down to the bottom, your arms just dont bend that way. you can alway drill an anti syphon hole in your return lines to keep from overflowing the sump if the power fails, or put your return pump on a computer battery backup. I did both with my new setup. still, when i did the what if the power fails for longer test, i didn't overflow the sump.

my tank is a 240 gal, and the return section of my sump it 14.5" wide by 24" long, and i keep the water about 9" deep. the overall height of the sump is only 14 ".