Ideally re-map first, fit exhaust then re-map again to see what gains you have.
If it was me, I'd get the exhaust fitted first, then get it re-mapped to work best with the exhaust rather than pay for 2 re-maps (unless the place you get the re-map done can fit re-map, fit exhaust then re-map again all in the same day!) because it sounds like your going to buy them at separate intervals.
I reckon get em fitted together if you can wait and then you'll be able to see what power your car did have(from stock?), and then how much more bhpzz the milltek gives ya!