Update as promised.....
I'm normally a huge sceptic when it comes to claims of huge mpg improvements, but I'm always willing to be convinced.
In this case, I'm glad I was.
The GT is habitually doing 2-5mpg more than it used to on my daily commute (I have half a dozen points in the journey where I measure the MPG, so its a semi-controlled test)
I filled up on the way to Motormech on Saturday and had the Terraclean treatment done. This involves holding the engine at 2,000rpm for ten or fifteen minutes, then a "soak test", then another ten or fifteen minutes at 2,000rpm again. When I drove away from Motormech, the tank average was in the mid 20s, as it had been running for ages and hadn't moved. A week's commuting later (circa 400 miles) and I've now used about 2/3 of a tank and the Trip A is showing over 50mpg.
On Thursday evening, due to leaving work late, there was very little traffic and driving quite gently I got a recorded 58.5mpg - my best-ever return on the commute. OK, we all know the trip computer is normally a pathalogical liar, but since the remap, I've found it to be spookily accurate - never more than 2mpg out and usually within 0.5mpg
If I can get over 50mpg as a tank average, I'll be well chuffed. I calculate that if I can get a 5mpg improvement, I'll get about 60 miles more per tank, which is clearly going to save around £7 per tank. At that rate, I'll be in profit in very short order
I'll keep you posted