I have the Titanium X 2.0, bought new in 2009. They did indeed come with 18" wheels as standard and like you I found it dreadfully hard. (I had a Citroen C5 before that so the Mondeo was a bit of a shock to the system!) I swopped the wheels to 16" with Falken tyres (nice balance of wear, roadholding and softness and not desperately expensive) and this transformed the car. It was so noticeable I felt the difference by the time I got to the foot of the street from the tyre dealer! I've kept my 18" wheels and tyres and will probably put them back on when I sell the car.

The only downside is the insurance. You are obliged to notify them that it's a 'modification', which is daft because the Mondeo Zetec has 16" wheels and comes with the same mechanical spec - just cosmetic differences in seat coverings etc. So you do land up paying slightly more on insurance. Bear in mind though that your 16" tyres will cost much less than the low profile 18" ones to replace so this compensates.

In terms of whether it's worth doing to make the car more comfortable, I'd say YES - WITHOUT A SINGLE DOUBT!

Apart from this minor drawback, the Titanium X is a lovely car. I've only had one single fault in 5 years - a DPF filter sensor, which you won't have on the petrol. Otherwise totally reliable and a joy tp drive.

Having one of these awful cars I can say without a doubt that changing the tyres will make it so much softer. Not great, but much better than new. But be careful, a set of Goodyear, for example, will make it very hard again.