The STS will most likely carry the same engines as the SRX that shares its platform and dimensions, which means base v-6 and optional northstar. The 400hp could come in the form of the V-series
A V6 Seville. I think it might actually be pretty popular, since its the "import fighter" and most imports have base V6s. Better gas mileage and some people like the smoother sound instead of a rumble. I think if they're gonna do that, make a new inline 6. Then you might start seeing sales.
It is clear that Cadillac is using the same look for every vehicle in their lineup...so I took a CTS put it in photoshop, and cut out sections, distorted them and reattached them to the car.
The platform for the STS is the same as the CTS which is why I used the CTS as a base. Extended the overhangs, rounded it out just a little, put on some larger wheels (increased the size of wheel wells too) and there you have it.