I'm not planning on doing this grout work with any block I have. No current need.

I was just thinking. I have read that sometimes the cylinder walls will sonic-check thin up at the top, which is where the pressure is highest.

True, it's also where you need cooling the most.

I have no experience with the stuff, I don't know how rigid, or strength-enhancing, it really is.

I agree, a true heavy duty block would be much preferable.