I would agree that if you want to reduce your running costs then diesel is the way to go. PD130 or PD150 cars are all fine performers, remapped or not!
I always wonder though how coost effective it is to change cars to reduce running costs....you pay any dealer a large margin on a 2nd hand motor, and then even with say 15-20 more MPG off a diesel, you need to do a hell of a lot of miles to claw back the cost to change!!
How bout an LPG converson to the S3!
And in everyday road driving, not soing crazy speeds, (sub 110mph) , not thrashing them, not trackdays etc etc, a mapped PD diesel with say 180hp will not be too far off a stock S3 performance wise. The concern will be will you miss that "specialness" that comes with an S3....there is just something about them, even if as stock, they have a few flaws!
A nice Mk4 non anni PD130 /150, with black leather recaros and a few toys would be what I'd go for if I were you. Immensely more practical than the very small interiored S3, and with a few choice spension mods, they really handle suprisingly well. Still a classy hatchback too and IMO a far far nicer interior space to be in than the Mk5 golf, or anythign this side of a facelify nice spec A3 / S3!