my GTR 33 has an under tray and i have removed both it and the foil of the rear wing, although around town i didn't notice any real significant difference, out on the highway was another story. the vehicles high speed stability had gon totally out the window, befor at 160 km/h the car felt comfortable on the road and didn't want to dart all over the road, it became more like a dodgey falcon wagon with stuffed shocks.
as of yet i haven't experimented with the placement of the rear foil angel, it is something that i do mean to get around to one day.
also of note is the benifits of smoothing out your under-carage is the gain in fuel economy, the slippery your car is through the wind, the less effort taken to propel it.
and with todays prices and my heavy right foot, i need all the help i can get lol