R-bumper, high speed stability

I'm wondering if anyone has suggestions on increasing the highspeed stability of an 854. If you'd experienced Combat Salmon Fishing on the Kenai Penninsula, you'd understand the need (and would refrain from flames about driving too fast, lol).

What I notice on my 854t is that at speeds over say 110mph, the front feels light, as though the factory bumper/spoiler isn't creating sufficient downforce. It's not *even* fun above 130. Would an R-bumper make any difference? Are there other methods of making the car stick better? I've thought of something for under the car to clean up the air going beneath it, but just dunno. What about toe settings on the front wheels? Suggestions?

tia folks!