I think having numbers to prove your case is generally a good way to approach things. If you can get objective numbers that really show and prove what you are talking about, you can swing most people your way.
If not, they are probably not the best of people to work with.
I want to reiterate that the guy was really good at his job. One of the best people I've ever worked with. He was generally pretty easy to get along with. But my lack of hard data meant I couldn't sway him on that problem, no matter how right I was.
That company had a history of not having metrics to work with. Every new sysadmin or db admin came in and asked for metrics and we'd shrug and laugh nervously. (I mean, there was a reason why hired so many new ones!)
If not, they are probably not the best of people to work with.