Having owned one for some 20 years, the Cessna 150's maximum altitude is limited by power, certainly not by Mach.
I also know this to be true. It is a nice situation, as the C 150 is not bound by a stated altitude limitation. I can speak from experience in saying that a carburetted 285 HP O-470F powered C 185 can reach 21,000 feet on Mogas (on the day I did it) but it does not have the power to remain there.
I thought about getting the 150 HP conversion
From my experience with 150HP 150's, I would generally recommend against investing in this mod. Yes, the power is there, and with the right prop, it'll be a better performer in a certain range, but probably at the expense of another range of flight - in which it could be less good than the original 150. It's a lot of money to get into a modified 150, when for about the same, you can have the equivalent 172 right out of the box, and have two more seats or more space, and range, for free.
The 150 HP 150 is nice, as a float or ski plane, to be flown with a fine prop, for more "up and out" performance, but little expectation of faster or higher. But the 172 (or 170B) will get you to about the same place for the same cost.