A 6.5' cadet weighs about 26# or a little less; a 2-hp 4-stroke engine probably about 30#; a 3.5 hp up to about 38#. A combination of your WM 8.6 ft dink and a 3.5 hp engine would come out at over 80#, plus the weight of the davits.
I can't advise you on an "acceptable" wt for stern davits + boat & motor on your Cat 27; maybe the other guys here can. Four-strokes are heavy; I once had a C-200 & 2-hp (two stoke) Evinrude that would carry my wife and me to shore & back no problema. The small dink could be stowed and inflated aboard my Pearson 30; could lift the engine with one arm onto the dink, etc. Those days are probably gone forever since one cannot buy new 2-stokes in the US anymore.
Frankly, I'm prejudiced--I don't like davits or the towing of dinks. I like 'em stowed below decks along with the engine if possible; otherwise, with the engine mounted on the stern pulpit. The reason is because I know that if I have to stow 'em, I can stow 'em. But given your choices, I'd rather tow than install davits for 80# of boat plus engine on a 27-ft. Others will see it differently, however. Heck, I still have not come to put davits on my Bavaria 38 even though the admiral brings it up every season along with a request for a hard bottom inflatable. To me, it's not worth having all that stuff on the back of the boat all the time for a few trips to shore each summer, etc. It's unattractive!! On a big boat-say 45 ft or larger, perhaps not; but on smaller boats yes--it looks to me like clutter.
