Difference between 1.5" and 2" drain in overflow

[christyf5]

So my 180 gallon was drilled for a 2" bulkhead. I figured since I was working with 1.5" (and I'd have to buy a full 12' of 2" pipe) I'd reduce it and do all the same plumbing. However my durso is pretty much under water above the elbow and makes a lovely sucking noise. I'm trying to make this tank as quiet as possible so for now I've throttled back my Dart pump to about half. In hindsight I probably didn't need the Dart.

Anyhoo, will going back to 2" pipe make much of a difference? I'd really like to run the dart full open or as close to it as possible for extra flow.