1M Arena Design gave me at least 5% boost in merchandising and gate receipts as well.
Overall, with the 1m arena desinger, level 3 merchandising store and tv&radio station, and exceptional PR manager. I consistently fill my arena 21k~ of 35.5k and 35-45% boost in merchandising (while having a bot like team). And this is all in Div. III
I don't know if this helps you at all. But I hope this gives you a little idea.
This also goes for my Div. III people.
Peace.