If the boat is rated for a 250, then my recommendation would be to go with the 250. Most boats perform their best when running the max HP outboards.
If you opt for less than that, the absolute lowest I would go is the 225.
In the end, it's not necessarily about top speed, but, as you mentioned, good performance without straining the outboard is important.
Also necessary to consider is planing time. A boat that's rated for a 250 will undoubtedly take a lot longer to plane with the 150 hanging off the back.
Not sure if this will come into play for you or not, but whether the boat will eventually have power poles or talons on the back is also a something to think about. They add considerable weight to the back end of a boat and will require more power to get the boat on plane faster.
All stuff to consider. But when everything is said and done, I would never recommend a boat that size with a 150 HP on it, especially when it's rated for a 250