I've had two Z cars with splitters in the past. Both were faster with them than without them, and neither had any issues with porpoising. We generate significantly less downforce than a sports racer with a flat bottom and front and rear diffuser sections.
Based on my previous experience, I believe your idea will work, but I'm not sure I'd put too much effort into it. You can't run a diffuser section on the splitter in either XP or EM, which would see this how to increase the pressure gradient and cause the bouncing.
I would suggest that you make the splitter hinged and able to move up when it comes check contact with the ground. This will save a lot of wear and tear.