Depends on what turbo is on it td04/equivalent will run out of puff at 280, larger one will go up to 350 but you'll need other supporting mods,
You could just change the exhaust & fuel pump, pop a panel filter in and get a remap and you'll gain the extra 20ish ponies but you'll have a big increase in torque and how it delivers it,
All depends on how far you want to go and how deep your pockets are,
If your happy with under 300 then about a grand will do it but if you want more i'd save get all the bits then do in one go as not much point getting a map then having it tweaked in six months for more mods