"Speed" records

There will probably be no documentation to whatever the top speed actually may have been. If you go by a search of train orders and sheets, you'll probably find that anything over 25 mph was rare, unless it was on the "speedway" between Alamosa and Antonito.
I will say that retired D&RGW Engineer Ben Greathouse once told me that he had run K-28's on stock extras where he could get them up to 45. If that happened once or more than once I can't say, but that was his answer to your same question. I believe he said that was South of Alamosa, but he also said he hit some of the curves so fast you had to really hang on, but that would be true at 25 in the hills.
There is some kind of "rule of thumb" that the top speed a locomotive may safely attain (track conditions notwithstanding) is the diameter of the drivers in inches to mph. This would imply that the larger K class engines might be capable of 44 mph.
Mike