Does Mercedes have anything in the way of turboed 6 cylinders? I know they have the AMG CL-A but thats not putting out THAT much more HP than a stock Q50. Unless they market it as "+70HP AND even better mpg!"
Right now the only one I'm aware of is the twin-turbo V6 that just came out in Europe with the new E400 and C400 models. This engine is to replace the current 3.5-liter M276 (an underrated jewel, I absolutely adored it in my E350 rental! Sound, character and power delivery reminded me of a smoother VQ!), as well as the E550's M278 4.6-liter V8. It's rated at roughly 330 hp and 354 ft/lb peak torque, and a assume a nice, fat, broad torque curve similar to BMW's turbocharged I-6 engines. But that's not nearly powerful enough to be the rumored Q50ER motor with the stats JdN hinted... at least, not without major surgery. The engine does sound very nice, albeit quite different from what we heard in the ER clips. Then again, this is in the ultra-hushed, refinement-first E-class application.
They're supposed to return to inline-6 engines with the next E-class, due in 2016 or 2017. That will be pretty cool to see after their 20 year hiatus from the configuration. It also will really make BMW sweat.

Maybe Nissan will get with the program and develop one of their own... nah, Ghosn is too cheap.
The CLA AMG engine is a turbocharged inline-4, and a sweet and beastly one at that. But that's not enough for the ER, which shall have to contend with turbocharged I-6 (M3) and V8 (C63, RS4) powerplants. I could see an ER version of the upcoming Q30 using the CLA45 engine, if they choose to make one, as the platform is shared with the CLA.