The cam profile changes at 5000 rpm, prob why you feel a sudden kick. And the characteristic of the engine is that they are a lot faster at high revs.
A remap is the setting in you ECU changed, like fueling/ air mix etc..... Speak to RS Tuning for more info. They are highly recommended on here for that.
To try make the pick up a bit faster, i would improve air flow. Like Sportscat or decat, exhaust, inlet match, ITG panel filter. Might help.
Edit:
Just relised Rich pretty much said it all.
I think the reason you think its sluggish at lower revs is due to the 5 k kick. This is well known and everyones does it ( although not so much with some mods).