I've always thought the forward facing turbo is the best orientation. What's been posted about real world experience jives with what I think should happen theoretically.
Two gains that come to mind with a forward facing turbo, both as a result of improving pressure ratios. Both turbines and compressors deal with pressure ratios (ie P2/P1). This is outlet pressure/inlet pressure for compressors and inlet pressure/outlet pressure for turbines. You see this on compressor curves, where they don't list the boost pressure, they give the pressure ratio, because that is what is actually important.
Gain #1. Eliminate the big elbow on the turbine outlet (ie downpipe). A straight shot out of the turbine should be a big help, just like other exhaust modifications. This falls into the same category as getting rid of the cat converter, eliminating restrictive mufflers, installing a bigger downpipe, adding better flowing pipes, etc. All of these changes decrease the pressure at the turbine outlet (ie there is less restriction from the turbine outlet to the tip of the tail pipe. Since the pressure at the tail pipe outlet is fixed, it is atmospheric pressure, that means the pressure at the turbine outlet *must* be less.)
The turbine provides power to the compressor side of the turbo, and some given pressure ratio is required to provide that amount of power. If you decrease the turbine outlet pressure, then a lower turbine inlet pressure (ie exhaust backpressure) results, while keeping the same pressure ratio and therefore the same power delivery.
For example - suppose the turbine needs a pressure ratio of 2.5 to provide the power needed by the compressor for the boost and air flow desired. If the pressure at the turbine outlet is 7 psig, then the turbine inlet pressure must be 2.5 x 7 = 17.5 psig. Now we do something to reduce the pressure at the turbine outlet (eliminate the big elbow, no cat conv, install a 3" downpipe, etc) and the new pressure there is 5 psig. The new inlet pressure, still at the same pressure ratio as before, is 2.5 x 5 = 12.5 psig. A 5 psig drop in exhaust backpressure thanks to the 2 psi drop in turbine outlet pressure! And as we all know, lower backpressure means more airflow into the engine, and therefore more power.
I think this was one of the causes of the big gains seen on Project X oh so many years ago, that set up had the turbos mounted in a forward facing manner.
Gain #2 - with the setup shown for the race cars, with the compressor suction sticking out through the grill or bumper, there is again a gain due to pressure ratio. In this case eliminating any suction side restriction (air filter, piping, MAF sensor, etc), will increase the suction pressure seen by the compressor. For a given boost level then the required pressure ratio goes down. In other words, it takes less pressure ratio to make 25 psi boost when the suction is open to the outside air than when the compressor is sucking through a MAF sensor, MAF pipe, air filter, etc. And a long MAF pipe with bends just makes that worse. A lower pressure ratio means the compressor takes less power to drive. Less power required by the compressor means less power that has to be provided by the turbine. Less power from the turbine means the back pressure required drops off some. And once again you see more power due to less exhaust backpressure.
So I think the forward facing turbo is the way to go. I would love to do that to my car, but as has been noted previously, I like bolt on solutions

Maybe someone should make a prototype and see what the power gains are, then sell a kit. What would we need... a passenger side header with the turbo flange reclocked. A new downpipe. A new compressor outlet pipe with an elbow to connect to the intercooler. And some arrangement for the air filter and MAF. Probably cost prohibitive for all that, unless the power gain was unreal. But if you are building a car from scratch, and you have to build such things no matter what, it makes sense to arrange everything that way. At least it does to me. I've toyed with the idea of a turbo 6 powered kit car, and if I ever do it, I'll be using a forward facing turbo arrangement.
John