Did this trip a few years ago... The Cabot Trail is amazing! My only regret is that I didn't do it both ways (ie - from S-N then turn around and go N-S). The route through the US is orders of magnitude better than going through Quebec. The roads through Vermont, New Hampshire, Maine etc. are stellar!
As for gear, I only take a mesh motorcycle jacket and pants with Frogg Toggs 2 pc. rainsuit. If it is cold out I just put the Frogg Toggs over top of the mesh suit... nice and warm! It is also pretty quick to don the rain gear when the weather changes. On one 20 km stretch in NB, the temp changed from 30C down to about 5C as I rode into an ocean fog. Ocean fogs are more like rain than fog... they are WET! For shoes, I wear Gore-tex lined leather workboots (waterproof and breatheable) and take along a pair of running shoes for whenever I am out walking around.
I always wear full faced helmets - given the statistics on head impacts (60 percent in the chin area), I can never understand why anyone would wear anything else. You'll probably find that extra helmets take up too much room anyway(given everything else you are going to be taking) so stick with the full faced helmets. I also take only 2 or 3 of pairs of the quick-dry wicking underwear (available at Mark's Work Wearhouse for about $20/pair). They can easily be washed in the hotel room bathtub at the end of the day and they are dry and fresh for the morning. The added bonus with these is that the fabric is a bit slippery, so you don't get any binding and bunching with your riding pants (and they minimize or eliminate "crotch rot".) With two up on a bike, you can pretty much count on doing laundry every 2-3 days anyway, as you won't be able to bring many clothes with you. Don't forget your swim suits... nothing beats jumping into a pool after a day on the road!
Have fun on your trip!