Now I know that the air filter, gubbins and remap can up the power from 180 to 215. But can the exhaust part of the upgrade add even more power? Or is it just a engine / exhaust sound thing?

I've had a quote for both and it comes in at just over 1200, so I'm saving hard at the moment. 

It won't add any meaningful power as the ECU is just working on torque targets. If there is less restriction, the engine doesn't need to work as hard to make the target torque/power.

What it can improve is responsiveness as the engine is not having to overcome the restriction in the exhaust.

Main benefit you will notice day to day is sound though.

I love my Mountune exhaust and that's on a stock map. :)

Would it matter if I for example had the remap bit done, then some months later had the exhaust done? I've saving up for both at the moment but I wondered if it mattered if I didn't have both done together

No, the ECU is intelligent enough to adapt anyway.

I haven't had any maps done, but can feel it is less restricted.

It won't be instant, it will take a bit of time/miles for it to detect the changes and adjust itself.

The catback exhaust makes no difference to the MP215 map what so ever, it adds noise, power gains are minimal as the restrictive part is still in place, eg the cat, you may feel it's faster due to the noise, It doesn't matter if you get the exhaust before or after the MP215 or not at all.

The catback has just come back into stock on their website.

Cat back does nothing but change the sound. I had all the Mountune parts mp215, intercooler boost pipes, induction hose all done before the exhaust. 

The other parts made a noticeable difference to how the car drove, but cat back was just the sound. I think sound people think it makes a difference due to the sound. Imho. 

I have the 215 conversion and have just added the cat back but only because it's step by step to further upgrades and i don;t have all the cash for everything at once 

It does sound better and at low revs especially (Mountune system) don't know if it's because i have changed my driving style but i got better MPG this week on an identical run to and from work normally it's 35mpg this week was 41mpg