I've done all of those things and found all the collectibles and i'm at 98%. I'm guessing that you either need to have all the upgrades unlocked or you need to complete the assassination/car theft missions in the saints book. What do you think?
I haven't done any of the assassinations or car thefts so i'll try getting those done and see if i get 100% but like i said i'm at 98% and it would suck if doing all those assassinations and car thefts only gets a whole 2%. And yes, thank god you don't have to do the stunt jumps or anything for completion, i don't think just going for stunt jump after stunt jump is very fun.