Replaced impeller and housing yesterday evening (did not test on the hose! lol), plus a few other items like greasing gimball and wheel bearings, etc. (messing with drive water plugs too... another thread in OMC section). Had it to the lake today, and it never got over 160F or so if my gauge is accurate.
We cruised at about 3k RPM so we really didn't push it. Did just under 20 miles. The new battery was great. I only killed it for an extended time once, but it restarted easily (had some hard restarts last year after sitting a while, which I think may have been related to cranking too slow).
Alternator's working too (per dash voltmeter), so that wasn't the trouble with the old battery. Hopefully this one'll last longer.