Any really good tuning job can improve fuel economy - suggest a 220bhp GTT conversion. Because the car has a much higher state of tune, it can be made to use fuel more economically..... this is perfectly true, until you start actually using the extra power in earnest.
A Wks conversion will probably see your car return 1-2 mpg fuel econmomy when you are driving very very sensibly. However, the minute you start to enjoy your conversion, fuel economy drops through the floor.
But this can be used legally a a justification method to explain your £3500 bill to your wife....
There is a school of thought that says you push your car hard to attain its desired speed, rather than pootle around, then get into top gear asap. Its better than making the car labour in inappropriate gears. 'Lazy' driving can be detrimental to fuel economy.
Cruise contorl can be very bad for fuel economy, especially on Briitsh roads. Theres too much stop/start on the motorways, too many hills and dips, none of which cruise control can take account for. So it ends up backing off just before a hill and then bogging down. Cruise is fine if you live where the roads are straight and level, like the states or somewhere.
I wouldnt be surprised if a decent large aftermarket intercooler would help fuel economy in the summer. But youd have to drive an awful long way to save the £400 in fuel...
The other thing you can do is run your car on the cheapest fuel. A standard MCS will be fine on 95 ron tesco fuel, which will save you 7p a litre.... thats whats impotant after all.