some great info here -- I also own a 2013 grandee and find that for extended soaks in cold weather, the 4kw heater is insufficient. the 30A side runs the jets, and since Watkins is greatly motivated to use inexpensive, readily available 50A hot tub disconnects, the 4KW heater is used. they wanted to stay at 50A to keep things simple, electrically, but there's no reason one can't wire it up for use with the watkins 6kw heater.
I'm grateful for the info shared here, and it's exactly right: put in the watkins heater, make sure the wire sizes are sufficient, make sure your sub panel is rated for at least 60A, and all the applicable electrical codes are met, and voila. I would only add that the wiring TO the sub-panel needs to be sufficient as well. for example, in my last house I had a 100A subpanel and 100A rated wires from the meter. I could have easily wired my grandee for the 30A heater.
the standard sales talk and under-informed will say that it won't make a difference but they fail to realize that a 50% increase in power to heating the water is a tremendous noticeable difference. I'm going to upgrade mine when I get a chance!
great idea and thanks again for posting.