The jon boat that I'm looking at is around 112lbs according to the manufacturer website. So I'm wondering would it be possible to attach front and rear straps from the boat carry handles to the rack bars to prevent it from shifting forward and backwards? I would think that would be a lot of stress on the rack if that boat ever shifted one way or another. Not sure how tight the rack feet actually fit against the car to keep it in place. I'm planning to use ratchet straps to secure the boat.
This is going to be the important factor. Ratchet straps will do a solid job of securing the boat to the rack, no worries there. If you also strapped it to the vehicle, the down force will also help keep the rack in place, but if you cant/don't want to do that, if one foot slips or disengages, would be the worst outcome. But that would be a rare event with a properly installed rack. I suspect modern racks are really well made. Stick with a known manufacturer and dont cheap out and I think you'll be fine.
As an alternative to the bumpers, if you really must have peace of mind, would you consider secondarily strapping it down through the windows? I think even that's overkill if you have a good quality rack.
The only piece of advice I would give is as you are securing each strap down, give it several good yanks back and forth to make sure it settles into it's shortest path, and it wont shift during transport.