I believe the general consensus is that, from a fog perspective May/June is better than July/Faugust. Of course none of this is cast in stone and fog is possible at any time.
While May/June would naturally be considered cooler than later, in reality you can essentially lose your summer weather on that coast at any time. May/June is also, of course, "less crowded" but that's also not an issue in those areas.
If 4 weeks is "c'est la vie" then that will help. Another way to maximize your west coast time would be to really gumboot up the inside, pulling long days and even perhaps an overnight if tides are more favourable then, I suppose one could get to say Pt Hardy or Bull Harbour in quite a bit less than a week.
From some of the accounts and guides we've read, the upper Island looks a treasure of neat hideyholes, quiet, private harbours and it would be a shame to go screaming by all those unless the primary object is to be able to say you've been "around".
We spent two full weeks in Barkley/Broken group and could easily have done more... in that area alone. Not much between Cape Beale and Sooke, so that's another candidate for a long dash.
Fishermen regularly use an inshore route around Nahwitti Bar.. but I think local knowledge here is key.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)