I just got a used Davy Crockett.  How high of temps can I expect realistically? Will it get over 400?

I believe it is able to be set for up to 550d.

Just an added thought, these have gone through numerous iterations and improvements over the years.
The first non wifi units had ‘air holes’ in the burn pot, this design had the flame directed at the left side of the unit resulting in the left being hotter than the right.

The later models had wifi and also had Air ‘slots’ in the burn pot like the other GMG models, creating a vortex like flame, resulting in a much more evenly distributed head across the grates.

The newest ones have the stainless heat diffuser and drip tray

Like always, these are ever evolving. 

The early wifi used the GMG guy on the hopper as an antenna, the newer have the antenna mounted directly on the controller.

I am not sure i have ran mine much over 450, Mine has the 2 piece drip tray. I know it runs hotter than the temp shows or at least it cooks faster. I have never actually checked temps. I use mine a lot as i am normally only cooking for 2 maybe 3 people.