This would be correct only if there's no skidding.
And even so, you'll have to make the following adjustments. The heat would be divided between the rim and the brake pads, roughly equally. And stainless steel, being a fairly poor conductor, the entire rim does not get heated immediately - only the portion of the rim that gets to be in contact with the pad (affected volume=rim thickness*pad width*wheel circumference)
This will give you an upper limit on the maximum instantaneous temperature reached by the rim. Radiation losses will be small and may be neglected.
You could also test this with some kind of strip thermometer, to confirm that it's not grossly off.