I've never used Goof-Off. But, if it's the same as Goo-Gone, I'm surprised it didn't work. The times I've used it on adhesives, it worked really well.
If that didn't work, my next approach would be isopropyl alcohol. If that didn't work, I'd try heat. With heat, either let the bumpers sit in direct sunlight for a few hours, or (cautiously) use a heat gun, while using my fingernail first to see if/how well it will release.
I've never had one of those deflectors, but the best I can remember, I thought those bumpers came with something like double-sided 3M adhesive, which isn't a tough job to remove. Do they appear to have been glued on with something else?