[JC]

If this helps at all, a mate of mine took the plastic undertray off his territory while doing some work in that area, and didn't replac eit. his temp gauge then sat on half. I advised him to put the undertray back on, and his temp dropped down to the "O" on Normal range. ie, it reduced his temps all round, and in theory should contribute to fuel saving as the fuel maps at lower engine temp (though not when cold cold) use less fuel than when the engine is running hotter.

I reckon you might be onto something, but take the 2l improvement with a grain of salt - maybe a 1/4 of that, and it will only take you a year's worth of fuel saving to pay for it, then you're in front (note that most mods you never recoup the "saving" from)!