So, one other issue that I have noticed, is drastic decrease in fuel efficiency of the C70. My 70L tank used to get 700km in the city, and as of late I barely get 450 out of it.

The car maintenance is up to date, and regularly looked at with all the recent issues - nothing has been reported to me in this regard - is there something else I should be looking at? I thought this would have cleared up with the throttle sensor reprogramming, and it the first 10 mins of driving I thought it was, but alas, it has not.

In the summer my 740 turbo gets 375 km on 48 litres, but over the last month I only got 320 km on 56 litres. Cold weather affects fuel injected cars MUCH more than carburated cars...

My wife's 850 turbo experiences similar reduction in fuel economy in the winter. It comes back in summer, just like my car.

Neither of us can or have ever come close to 10l/100km like you used to get.

Exactly what Matt said.
My C70 gets about 450 km per tank if I am lucky in the winter.
Short trips have a huge impact because of the very rich mixtures when the engine is cold.

Some things to check are: is your thermostat good, is the temp gauge reading at about the 3:00 position and steady when the engine warms up? Does your car have any DTC's (diagnostic trouble codes) mine was listing the ECT sensor (engine coolant temp) and replacing this helped my performance and economy.

Don't just toss parts into your car, make sure the part is faulty.
Realistically 450 per tank sounds normal to me.

Darth Shaggin Waggin gets about 600km on 70 liters in the summer but that's down to a little over 500 now...

The quickest way to warm the car up (and therefore operating most efficiently) is to let it idle for a minute but start driving it pretty much right away, nice and light on the throttle.

Is that comment directed at my driving specifically?

Actually I think it was probably directed at me

...i don't think the s70 ever gave me 700 city per tank. that would be nice.