[I think somewhere someone missed read my post I put "A sump would be a wasted expense on such a small tank". I never said it didn’t have perks.
I agree with Mitch it would increase water volume. But, work this equation buy a new 29 gallon sump add the baffles, live sand, return pump, lighting (if you include a refugium), etc... and you just doubled your investment.
For an experimental tank which earlier today you told me was a temporary set up to get use to the idea and concept of a marine aquarium... "you live in an apartment and plan to upgrade to an in wall tank when you move into a home".
Don't waste the money on an extra 29 (14 or so gallons after the baffles and all) it is not worth double or even tripling the cost of that tank.
If you want to go large sell that tank and buy a 55 gallon aquarium. You can even get one with a built in over flow and sump... it will be more expensive but the fruits are worth the added labor.
Again, I agree with Mitch’s statement and the best reason if you should choose to add a sump is the extra water capacity...but, let me reiterate it will not be enough to significantly improve the water conditions!
As I mentioned during your visit the primary reason for a sump is to hide all your “toys”. By adding an overflow you automatically defeat that purpose!]