Am I using Shannon-Hartley Theorem and thermal noise correctly here?
Looks reasonable to me. 

Dont forget that you are calculating the maximum possible THEORETICAL performance for that channel assuming it is in fact thermal noise limited, at VHF and up that is not usually the case. 

The radio front end on a cheap chipset at UHF will not have particularly state of the art performance, and getting real modulation and coding performance anything close to the theoretical limit is a big ask and is not likely to be possible using such a simple coding scheme. 

They have in effect traded information bandwidth for simplicity, low power and some interference rejection, not a bad trade for the intended uses (Some form of COFDM would have better channel coding efficiency but needs linear amplifiers in the transmitter for example, much harder to do).