To be honest I would probably get service, cambelt, waterpump & swirl flap removal done together. Maybe add EGR delete to the mix too especially if your existing manifold is choked up with soot. Then see how much better the car runs before deciding whether or not to go down the remap route. There shouldn't be a dead spot under 2.5k rpm, that indicates there is a problem somewhere, its not a characteristic. I just did a similar job on a GT 1.9 JTD and we got 400ml in volume of soot out of the inlet manifold. It drives like a new car now, it feels like it has just been remapped and it hasn't!
Remaps are great, you can get a good 190-200bhp from this engine but it does put a lot of stress on the gearbox, they are pretty fragile things are the M32 6-speed boxes. Remapping a car with issues is bad practice, it makes it harder to diagnose faults (like your flatspot).
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