Strength will depend very much on design, especially in the area of attachment points.
One dissadvantage of PP in bumpers is that they need to be painted off line (as the PP will not withstand the temperatures in a normal paint oven), or at least had to when I was involved about 10 years ago, but this I expect would not need painting.
Another dissadvantage of any plastic, but especially PP is it's high co-efficent of expansion, especially when compared to steel. This means that bumpers must be designed to hide the extra expansion vs the steel body. As I guess this underbody cover will only go from chassis rail to chassis rail, and is mostly out of sight, I expect that this would not matter in this case.
I am presumeing that this is aero, not protective, as I would probably advise steel for a skid plate.
If it is a skid plate, and was to be plastic, I wouldn't consider anything other than super tough or glass filled nylon. Nylon would also be my second choice to PP for an aero only component, that is if the environment was to severe for PP.
If you carefully choose the right grade of PP/EPDM for your trials, you should be able to pick one with similar shrinkage to nylon 6, so the mould could work for either material, giveing the luxury of alternate materials to test in the field.
Obviously, this is all pretty well off the cuff nonsence without a lot more detail about the design, function and environment of use.
I would prefer to refer to Noryl GTX as PPO/PA alloy as this is suposed to be a technical forum, not a marketing tool.
As I expect that ultimate dimensional stability, and paintability are not an issue here, I see no advantage in PPO/PA alloy over PA.
Regards
pat