[SPQR M3]

So I'm installing an Akrapovic Evolution system on Tuesday. I've done a fair bit of research and looked through a few sites, read some articles etc. My specific questions are not really answered anywhere though, so hopefully some of you can help:

1. I currently have the Eisenmann Sport. I know the Akra system will be louder, but does anyone have any direct experience with these two systems as a comparison?

2. I've ordered the Delete-R module. I know this will prevent the CEL from being thrown, but apart from that, is it recommended to retune the car? If I do not do a tune, what is the likelihood of any damage being done to the engine in the long term?

3. How will fuel economy be affected?

TIA