Hi Ian,
You are not the first to be dissatisfied with a remap, although there are many who are very pleased with the car after it has been done.
I have not been tempted to have a remap on my S Type 2.7 diesel as I find 208 bhp quite sufficient and I do put my trust in the engineers who designed and built the engine. I think they do know a bit more than I do and probably a bit more than those who do the remap.
The main reason I do not like remaps is that in 1963/4 I was driving a Honda motorcycle which was capable of 100mph plus. A friend had a motorcycle of a different brand with a top speed of circa 95 mph, and to compete he had it retuned to a high level, and on our first day out on a local dual carriage way [no motorways and no speed limits then] while I was doing a steady 90 mph he passed me at about 110 mph - and I couldn't catch him until his engine blew up!
As Virgil put it "Hunc et olim meminisse juvabit," which is loosely translated as "he who laughs last laughs best"
Regards,
Peter.