Sounds great Rob, I'm near doing it on the touring which also has grey seats.
I'd ask
1. Can they do double stitching in M colours blue/red. It makes a dark grey or black seat look good. I imagine it'd work on Lt Grey too. Definitely an easy job to do at home with a pair of pliers and the right torx wrench/socket, just need to take care undoing the seat belt adjuster clips and any electrical connectors underneath before removing the seats.
2. Consider getting a pair of sports seats first if you don't have them and don't mind a tight seat. The investment is worth it before changing the leather- less so after
Wisheadliners look good, its a pity international shipping is so expensive.
Carbon wrap makes a massive improvement over that POV woodgrain shite. My 540i/6 has carbon finish standard, its much nicer but I reckon carbon fibre wrap would be better than the glossy finish. Add an M5 shift surround if you have a manual and the interior'll be sweet as.
Don't forget to post before and after pics!