Difference between 1.5" and 2" drain in overflow

[shrimpchips]

According to here, the difference in gravity flow between a 1.5" drain (2100gph) and 2" (3300gph) is 1200gph - fairly substantial. Now, I don't know if those numbers are 100% accurate or not, but even if they're off by 25%, I'd say there's still a significant increase in drainage using the full 2".