I've had a few leons... 20VT Cupra, 110 TDI SE, TFSI FR, FR TDI 150...
Fuel economy on TDI 150 is very good when driven sensibly (I even managed over 60mpg on a long journey). In gear it is quick although off the line it is a bit slower than the 20VT. I think over 5000 miles I avg 43/44 mpg according to the trip computer. I work on the basis that the TDI was about 15mpg better than then 20VT or TFSI
It all comes down to what you want and need... even though the TDI 150 was quick I missed the petrol style pull towards the redline, the diesel sounds very tractor like when cold and causes a few more rattles in the car IMHO. I changed my TDI for the Mk2 TFSI FR as I only do 8-10K per year now, but have just "downgraded" to a 04 plate Cupra as I need some money towards a house purchase.
The tax on the TDI is dirt cheap but the insurance was only £10 or £20 cheaper, also it chews front tyres but maybe that was my right foot! Servicing is slightly more expensive & diesel is about 5p per litre more than 95ron. Factor all this in and you might not save as much as you think.
You can go down the chipping and tweaking route but remember why you are buying it to save money....
How many miles do you do per year? What is your budget?