Finally changed the spark plugs today at 37k km. The rear ones - close to the firewall - were extremely hard to remove and were seated very tight, and so were the other. The plumbing around the back ones made it even harder. I have bursitis in my left shoulder and didn't help in terms of strength. Even with the tool the rear ones wouldn't budge. Had to use an improvised tool in the form of a wire, and looped it under the neck of the coil and pulled them out. This was very easy. I took the whole process slowly.
Here are some pics of the spark plugs removed. I am not an expert on spark plugs and their wear. Hope someone can comment. They look in pretty bad condition to me.
Went for a drive afterwards but couldn't feel anything special.
2007 E92 6MT - Silverstone II with /// Performance Exhaust with ACM Test Pipes and BPM Sport Stage II Dev-Tune - Proper engine warm up, 12-monthly oil change, Castrol TWS 10w-60
2011 F25 X3 3.0d