A 210Bhp Megane will be faster, And I have always preferred diesels anyway.
Even now when I rev past 6k I think my cars going to explode!
So you're worried about revving past 6k, but wanted to spend £3k on a turbo kit.
Diesels cars are not fun to drive. I moved from a 182 to a VW Jetta due to mileage and comfort.
Unfortunitely my jetta needed a new turbo a month after purchase, so had a hybrid turbo upgrade and a custom map (only did the upgrade because it worked out around an extra £300 over having a rebuilt standard turbo and a non custom map). It's now running around 200-210bhp (140 originally). Not sure what the weight difference is between a jetta and a megane diesel, but my jetta is at least a second slower to 60 compared to my 182, and it's not until 70 or 80mph that the jetta feels as fast.
It also corners massively slower and the brakes aren't anywhere as good.
If you need a diesel then go for it. But if you want a faster car, which is what you seemed to be looking for then a diesel megane is not the answer, even mapped, it'll be much slower than a standard 172 in pretty much every situation.
Also if you so go for the megane diesel, look into clutch and dmf issues, as they may or may not be able to handle the extra torque from the remap.