i havent read all the comments on here yet but in response to the OP,
i had an MGZR TD 115 with a decat and mafam box on it, thats all it had. did about 40mpg and its mid range grunt was impressive, 3-4K it woulsd accelarate (after some serious lag) pretty hard. I loved the car alot but though it was quick midrange it wasnt useable really and starting it up i dreaded as it just sounded so rough and awful, sounded like it had serious diesel knock but was assured it was fine.
Had several friends with 306Dturbo's and my only comment can be though you hear good tales on the internet and hear of people with good ones none of the ones i have seen have been much cop. standard they are guttless but will do a genuine 55mpg, but turn the pump up and they rattle themselves to pieces at idle, every copper pulls you as everyone behind you has developed emphysema and like my old MG, the power delivery is useless. Also, as a result of the pump being turned up by someone who didnt know what they were doing the headgaskets usually fail, alternativly, they adjust the boost way out of line and something else breaks.
Remap wise, a few of my friends have had cheap remaps done, the power has been noticably increased but a few have had turbos fail, usually on VWs i will add. but usually not bad. Having spoken to a few mappers they have all highlighted that to create a map you need to have the car on a rolling road, and every car being slightly differant after a few miles in regards to wear and filters, oil etc they all respond differantly. As well as this, any modifications you have on the car will alter the peramiters of a map. long story short, if you get a plug in and play remap, how do you know its not just a generic map from a ... 307hdi? most diesels have the same ECU so you need to be careful.
My own experience, i have a C4 VTS HDi at the moment as a trog about. It had 136bhp as standard, a dyno revealed 12 of them had escaped. after a remap, with the boost at 21psi it made a sound 191bhp and just shy of 400ft/ib. does around 35mpg and is great! being a 16v diesel is isnt quite as breathless higher up in the revs so drives more like a petrol.
But still imo, my VTR is the better to drive, i get 50mpg driven steady and when its not its a right hoot!