The high speed buffet boundary of a Cessna 150 is certainly something you want to approach very carefully
If by that you mean that the aircraft doesn't have enough power to reach its high speed buffet boundary then you may well be correct.
But as altitude increases, the two boundaries will converge with maximum speed somewhere between the two. And as darkroomsource has said
The higher you go, the closer the stall speed is to the maximum speed of the aircraft.
This certainly sounds like being the case where full throttle straight and level flight results in the stall warning sounding.