Interesting talk. I know one operator who once told me that people don't expect much, so long at they get pressure when they pull the trigger they're happy. He's partly right, but he is probably leaving some revenue dollars on the table.
Personally when I drive down the road and I see two gas stations, and if one is newly renovated, I'll probably go to the newer station. Likewise, why do large chains, be it restaurants or retailers renovate or modernize the look of their locations, why because it tends to generate more sales/traffic. Everything being equal, I would think locations that are clean, have newer equip., and everything works would outperform a nearby competitor.
But back to what this thread is really about, what is the optimum expense/reward amount for improvements or the amount a wash is attended. I don't think there is a real answer, each location is unique. As suggested you have to try different things and see if they work.
Personally I never really advertised for years because I have a high traffic location, was making decent returns and I was happy. 18 months ago I decided to try some advertising in the local paper, it wasn't cheap, but revenues increased 25% and are holding. I don't want to know how much money I left on the table all these years.