I ran two of those 20HP Walker GHS models with 48" decks. I never felt like they lacked power to do the job. I cut a lot of thick Bermuda and St. Augustine with mine. most of the time I used mulching decks, but in the spring and fall, I used the collecting decks for big cleanups.
The 20HP is plenty, and that engine does not drink too much fuel compared, to engines like the 25hp or 27hp kohlers.
It does a beautiful job on our southern grasses. I did have to slow down sometimes on very thick St. Augustine, if it was a little wet. It give a great finish cut and the clipping just disappear. It is important to keep the blades really sharp when mulching.