Question how u understand the 5 min boost limit?
For me its like for the 109, u can use it for "5 mins" depending on temperature, after cooling and reaching normal temps and one can use it again.
Sure it stresses the engine, and all the Pilot has to do,is to tell the groundcrew that he has used it, so that they can decide how to inspect the engine.
i am referring to these both links: have a look in it ;D
(edit: merlin packard 1650 / technical manual for merlin 66 67 70 71 76 77 85)
http://aviationshoppe.com/manuals/v-...rd/merlin.html
http://aviationshoppe.com/manuals/en..._76_77_85.html