When you set those engines up you had to put a temp probe into the intake, then you get the chart from the maint manual, run the engine at full power and then match the temp with a boost setting (with regards to the chart) that gave you 350 HP.
That's why you would see different boost figures on T/O.
Boost doesn't directly relate to HP.
Changes would be due to worn components, density etc.
That's how I understood it.