How did you determine how much strength to add Vic? Did you pass any of this along to the camper builders?
As Frank just said, lifters are being used by the factory now. 30# is stock, I believe. Like Frank, I installed mine myself. I've been using the 80 pounders for 30+ camping nights now, sometimes with the canoe up there, sometimes not. No visible stress yet.
Downside of my install is that when the canoe is not up there, it takes some effort to pull the roof down. Using the fan to suck air out helps with that.
I'm not sure that any changes to the camper builds are necessary to "support" the lifters.