Could it be the fuel filter in the tank?
not likely with the miles he has, and the fact it runs well when warm, and does not exhibit higher RPM stumble issues.. fuel filter really shows up when trying to maintain constant higher speeds, on longer runs, and is evident by variations in performance "when forced to flow more" than at idle.. The only thing I've found that is really clearly effecting idle speed vs off idle, especially cold, is air filter, and poor quality fuel. The filter had/has little to do with the quality of the fuel.
I did mine this summer, never had an issue, but just did it due to the time it spent idle, and differences in fuel from Va, to Ohio.
probably could have let it go till 60k, but as I was doing the test on new MRP flash, and going to the Nationals.. I did it pre-preemptively..and as I waned to be very precise in my testing, I wanted a perfectly clean system to evaluate the changes... I don't like doing 5 different things, and trying to figure later which really made the difference.. if ya get my drift; so I won't be doing it for another 30k miles, or much more.