I'm at 95% complete now, just need to complete the stunt jumps, under the bridge, knife flying things to do and 1 street race. I've also got a couple of misc tasks which don't seem to register - for example I've robbed a store with both Michael and Franklin but the "rob a store" misc task doesn't get completed on the R* social club checklist, maybe there's a specific store you need to rob or something. I've also got 1 strangers & freaks mission to complete but apparently that doesn't unlock until a while after 100% completion.
There's probably a ton of stuff to do even after 100% completion though.