I've posted this elsewhere in reference to an excellent mini revo custom build, it may be of some benefit? Apologies for being lazy....
"I see there are 4 basic elements to an open wheel design...1st is the splitter, 2nd is the body, 3 is the rear wing & 4 is wheels
You can adjust everything except the wheels fairly easily & this is the splitter function...
but in your case I think something more exaggerated in terms of length & perhaps even reducing the high pressure area by smoothing the splitter to body area - which increases the downforce...& then if you need to, you can reduce this effect by reducing the size of the splitter. I hope that makes sense?"
In most cases, the splitter can be mounted directly to the chassis via a simple CF or alum plate using the existing diff screws
I also think a 1/8 rear wing might be too aggressive & reducing the near vertical elements will work well to decrease drag but maintain sufficient downforce. Combined with the enlarged vertical end plates, it should work perfectly with some fine tuning