I wish you luck. To the best of my knowledge none of the OSX releases allowed you to make those changes. However, some clever person or persons would eventually post a work-around to this problem. The solution generally required going booting to Recovery, entering a command so that you could edit and then editing the correct .plist to change the default "3" [seconds] to whatever you like. Apple 'may' have embedded the timing somewhere else, or somehow hid that default.
In any case, since work-artounds have been created for the prior releases, my guess is that more than a few customers have wanted this option and that Apple definitely does not want us to have that.
The developer is right; the customer it either wrong or doesn't know what is good for them.
Unless that clever person/s posts a new work-around, I'd say you, I and who knows how many others are S.O.L. I truly doubt Apple will change their minds. Do as they say, not what you want!