I have driven both and you can definitely tell the extra torque from the bigger six in the S90. With the weight difference, though, the power-to-weight ratios are practically equal. I think what makes the 850 feel like a quicker car is the difference in transmissions. The S90 felt geared toward luxury, very smooth (very nice), and unless you step on the gas pretty well theres not as much of a difference between the Eco and Sport mode. At least in my experiences anyway.

If it were me, I would either take the engine from the S90 and put it in the smaller car or just buy a T5-R... But thats just me :)