I think the boy is confused
If you get a diesel for economy, then remap it, then drive the balls off it, it will not be economical at all and you will have been better off keeping the TS.
If however, you buy a 2.4JTD, keep it standard and drive it gently...(not what you want to hear I'm sure
you CAN get close to 50mpg as that is exactly what Mrs A gets out of our 156 2.4 Sportwagon, week in, week out. She does over 700 miles every working week, so we have a fair idea of consumption.
She drives with a very light foot, which is why the front tyres lasted almost 40,000 miles and the rear ones 82,000 miles.
I fitted the cruise control to see if i can get her to achieve over 50 mpg
It won't matter which car you get next. If you drive it hard it will use juice.