To clear this up all these turbo upgrades are not the same. Renntech chooses a custom Garrett GTX2870r based setup. AMS also uses a GT28 based setup. Weistec goes with a GT22 CHRA and exhaust wheel. Tuners and users chimed in with their opinions and a tuner from the BimmerBoost.com side stated his belief that the Ball Bearing CHRA of the GT28 is superior to a journal bearing based setup as in the GT22. Garrett's own position is such:
Garret Turbo: "The journal bearing has long been the brawn of the turbocharger, however a ball-bearing cartridge is now an affordable technology advancement that provides significant performance improvements to the turbocharger.
The ball bearing cartridge gives better damping and control over shaft motion, allowing enhanced reliability for both everyday and extreme driving conditions. In addition, the opposed angular contact bearing cartridge eliminates the need for the thrust bearing commonly a weak link in the turbo bearing system."
Now the Weistec choice is the GT22 from the Garrett gasoline turbocharger family which is a newer design over the GT28 and they believe it is the better choice. This is due to lower inertia and its more compact size which allows it to flow close to the GT28. This is their position: http://www.weistec.com/turboupgradefaq.html
Weistec Engineering: "It is slightly smaller, but more efficient, and proof that bigger is not always better. The process of adapting the GT28 into the factory Mercedes exhaust housing requires extensive machining of the volute profile, an increase in radius and flow width to install the larger, older exhaust wheel. This reduces the A/R of the already small housing, as well as reducing the yield of how much exhaust gas actually drives the exhaust wheel forward via the volute.
Machining to fit a larger wheel into the same exhaust housings effectively reduces the AR even further and with it, the flow potential and turbine efficiency of the GT28. The result is operation at an even lower level of efficiency than the OEM configuration at multiple points. Although the wheel is larger, less exhaust gas will flow through the turbine. This is reducing the efficiency of the system. In the performance world, efficiency is power, drivability, time to torque, response, back pressure and again Power!"
There are two different perspectives at work here and both have merit. When going to a GT28 based turbo upgrade it will require running new oil and water lines. Since the Weistec setup is identical on the outside to the OEM setup a dealer will not know the turbos are upgraded which is handy for those who are concerned with warranty work or like to keep their mods hidden. A GT28 based setup will be noticeable. Again, pluses and minuses here.
Each company here chose the setup they believed was best for their customers. All are good choices and are an upgrade over the stock turbos. It would be great to have a shootout between all of them but due to distance that is not possible. Keep in mind these upgrades use the stock housings and manifolds. The M157 is a fairly tight fit which varies based on the chassis it is in. This motor will see much more development over the years.
For now, let's enjoy having options.