There are a million threads about this mate, did you have a search ??
To sum it up-yes.
Re mapping a OEM you will get better drivability from it.
Mapping a modified car will magnify the effects massively.
Remap's or tweaks with roughly cost about £250 so it's an expensive thing route to go down mapping the car first then getting tweaks everytime you add a breathing mod.
If I were you I'd get all the basic breathing mods first, (filter, cat back) then get a header, decat and ECU and get it mapped at a later date.
That's the tried and tested method.