I will endorse everything said above. The difference of a sigma with 7000 miles on the clock compared to new is unbelievable. One of the cheapest things for a power gain is fill up with 98 octane - you'll be surprised what that does!
Start with the low cost mods and see what that does and then work you're way along the list if you need to.
As well as the 44mpg enjoy the cheap servicing and road tax costs too - it's the most fun with your clothes on!
