Hello, I hope the following is of assistance.
Although the 1.6 is new to the Mondeo the original DV6 engine was fitted in the Focus from 2004, the DV6 in the Mondeo is modified compared to this engine although the same engine is now fitted to both the Focus and Mondeo so it is a proven engine. The Mondeo since 2001 has always had 115 PS diesel engine so the power to weight ratio is no concern. Having driven both cars there is a noticeable difference between the 2 power outputs even when the early 115 PS was the 2.0L DW 10 engine rather than the 1.6 used currently for this power output. The 1.6 is used now instead to lower emissions and therefore improve fuel economy and put the car in a lower tax bracket especially for company cats which make up the majority of Mondeo sales.
Originally the larger the engine capacity the more torque the engine produced but due to advanced technology this has been changed giving the advantage of high engine torque from relatively small engines, the 1.6 DV 6 in Mondeo produces 270Nm of torque at 1750 to 2500 RPM, the 2.0 DW 10 produces 320 Nm in 140PS form (it only produces 300 Nm in it's earlier 115 PS form) the higher the power output and torque the better the acceleration but you get the loss in fuel economy.
Some people want the best performance they can have from a certain model and don't worry about fuel economy or the associated costs, they may also use the car for towing, these would buy the 175 or 200 PS model, other people who are concerned more with fuel economy and associated costs rather than performance plus perhaps only use the car for towing very occasionally would opt for the 115 PS version, the 140 and 163 versions are a middle ground.
This is very much a personal choice as each engine has it's benefits and draw backs but the 115 PS DV 6 is a capable engine and can manage the weight of the Mondeo easily but obviously won't have the performance figures of the larger engines but the fuel economy is better averaging around an extra 10 to 15 MPG compared to a 140 PS.
Most people who have tried the 1.6 have been surprised by it's ability especially it's fuel economy achieving over 60 MPG average if stop/start equipped which is very good for a car of this size.
I hope this helps in your decision making but if I can advise further let me now and I'll reply as soon as I'm able to.