The VP+EX uses 978 Out on a seperate antenna. Three In/Out antennas in totally, well 4 if you cound the GPS and 5 if you cound the AutoSquawk. The key antennas 978 Out, 978 In, 1090/ES In. The 978's are timed so the 30w "out" on 978 is only for 1ms each fuel second.
The Pi is not bad for an In solution, but unless you have an ADS-B out, you are not seeing anywhere near the pull traffic picture. Only other ADS-B Out planes and "piggy backed traffic. It takes the ADS-B Out to trigger the Full Traffic for your "Service Volume" a 15nm radius and +/-3500 area around the ADS-B Out Client location. Full Traffic includes Mode C, Mode S, 978Out and 1090/ES Out.
The transmit and receive works great on fiberglass planes. I've sold many of the VP+EX to GlassAir, EZ, CT, and other composite planes. It's not really for the isolation so much as for the interferrence of metalic surfaces. The Transmit "Out" is pinging the ground station and the best place to reduce interference is the buttom. The Receive "In" can be above or below, though I recommend splitting them somtimes depending on real estate, aircraft type, speed. The 978 "In"receives ground station Traffic and WX, so the bottom is the optimal place, but the 1090ES "In" is really aircraft to aircraft so the top/bottom is fine. Obviously above is best fo the GPS. Metallic surfaces attenuate the 978 and 1090, so reducing that shadowing is best for long distance RX and TX.
If you don't have ADS-B Out, I recommend getting it and you'll be amazed at how much more is out there. I hope this helps.
Ps: Bought the aluminium case for under $10.