Are all diode readers going to produce the same results?
I doubt it. As for advice on specing them, check maxim's technical document.
Keep in mind though that Andrew (Millennium Thermal) said his die simulator (same design as JoeC's) typically is about 15% higher in C/W than JoeC's. I would think that the diode readers should all be that close
I should also note that some heatsinks never performed as well for me as JoeC predicted, and others seemed to do a lot better. I found the Alpha PAL8045 to be substantially better than the Swiftech MCX462 with a variety of fans, and I found two of the Dynatron heatsinks equipped with the 38mm thick Delta fan (45CFM) to both do a good bit better than Joe saw with his simulator. For the Dynatrons, I really think I was seeing some secondary cooling due to the enormous amount of air around the socket. For the Swiftech vs. Alpha, I am chalking it up to Joe's simulator not really being designed for 4hole mounting (something about wedging a piece of wood in place that just doesn't sound good to me).
At any rate, I feel comfortable pointing people to Joe's huge table of heatsinks, as my testing and his seemed to jibe pretty well in comparative terms.