Tough answer! In CA and most other human habitat areas 2 KW will do the job. Here in the NW, when the fronts come through in waves and rain by the bucket -- I've been in total whiteout with 2 KW. 4 KW would be better, and even that might be a stretch. I've been doing radar, marine and airborne, professionally and amateur (ish) since 1944 -- and there just isn't a simple anwer.
Money talks! If I didn't care about the green machine I'd buy the E-80 and 4 KW head. Why the E-80? With its separate video processor it is much faster. The screen is brighter, and it will take the Platinum... none of these reasons make it better operationally for a small boat. Why 4 KW? The latest C-machines (and Furuno, and... and) are many times better than just a few years back -- back when I was a dealer in these gadgets. The more power you have to punch through the crud, the better off you are. If'n one had any sense he wouldn't be in that kind of crap anyway!!!
Bottom line I think. If you can afford the E 80 and 4 KW do it, because if you don't you'll always wish you had. From an operational point of view -- the C-80 and 2KW power head will do the job in most areas, and at least 95 persent of the time here in wet country.
HTH,
Dusty