There is a maximum storage temperature (150 degrees) in the datasheet's
absolute maximum table. Would I be correct in thinking that this is also the
max Juntion Temperature which I would use with the Junction to ambient thermal
resistance to de-rate the power dissipation with temperature?
The absolute max junction temp is 150C. The operation is not guaranteed at 150C
junction, it will just not be damaged up to 150C junction. You should try to
limit the junction to 125C to obtain datasheet performance in operation.