I've got the front splitters and personaly i think they finish the lower front end off, i often park next to another white RCZ in my local Tesco that does not have the splitters and from the side they do make all the difference to the look of the front end, i personaly think the lower front end looks a bit unfinished without them, like as if the splitters should be on there in the first place from build.
Not sure how they would look on a different colour, as being polished black they stand out more against a white paint finish.
I also have the black Solstice 19" wheels so i think the whole car looks a bit lower anyway compared to an RCZ on standard rims. Optical illusion i guess.
I have'nt had an issue with speed bumps or low kirbs (yet) so fingers crossed.