and there is actually a small gain in weight with the turbo over a stock muffler. but it is definitely offset by not having a huge cast mani up front. a net loss in vehicle weight overall.

You are correct and this is why sizing of the hot side is so important. You have to go smaller than a front mount to make up for that.

Since the exhaust gas will be cooler and thusly denser, why would you need a smaller turbine than if it was mounted in front? Just curious.

Because they also have less energy. The equation for energy a turbine can extract includes incoming air pressure, temperature, and mass flow rate - but not, directly, the density. Less temperature will drop the air pressure and temperature while leaving the mass flow rate the same.

Lot of money for a little play, don't you think?

Although I have a rear mounted turbo, id have to agree with the nay says in that unless yu go the pump to push any decent sized turbo, front mount is the way to go wit a 4cyclinder. may be a V6 with a properly sized turbo. Its hard enough to get a rear mounted V8 to run efficiently with wrapping exhaust pipes, proper sized turbine that balances flow and spool without sacrificing to much power or drivability. I got a T76 on my LS1 trans am and spool doesn't kick in until about 3000rpm and I have a 3200 stall. I have the grunt of a V8 to make up that difference. I couldnt imagine trying to do the same with half the cylinders even with a smaller sized turbo. you would have to have something in the range of a 50mm turbo if not smaller just to have ay fun out of it.