That is impressive, and does reflect the power of the K5 sensor when properly used. I have found that I can get decent prints, ones that satisfy my need well up past ISO 6400.......getting the right exposure is, as mentioned, key to the end result.
I have an 8x10 of this one, looks better than it does on screen.....
1/200 (speed is also key) F7.1 @500mm ISO 8,000 I think I was about 40-50ft away from the target.
Overall, I have been well pleased with the new abilities presented by the K5. It certainly expands the opportunities to get a shot without flash.
Best Regards!