That elastic stuff sounds good... however, which ever filler you use if anything is going to crack/fail first it'll be the paint so there is no best way to do the job you're after. You scrape the bumper (making it flex), it'll crack the paint. In hindsight, i'd have kept the splitter as a seperate item, sprayed it and fixed it on. That way, when you damage it, it can be resprayed individually.
All i'm saying is, don't hit any speed bumps!! And yes, the elastic stuff it probably about the best you'll find.