Hello guys, I have SM300 MC with 8400 movement. I bought it 15 months ago and noticed it began gaining 4-5 sec everyday for 2 weeks observation. This watch is NOT master chronometer but just chronometer still COSC certified which it should keep between -4 to +6 sec/day deviation. So I decided to bring it to Omega boutique where I purchased it. They send it to the Omega Service Center and decided that my watch will need a full service with cost of no charge since it is still under warranty. They gave me the estimate service time of 6 weeks or earlier and it actually took only 4 weeks. I received my watch and noticed its rotor seems moving more smooth than before (it might be my imagination) and also they provided a red omega travel case (like other brands provide a pouch after service). On the after service paper, accuracy of watch was +- 3 sec/day. I wore my watch over night to see how it goes. surprisingly, it only gain 1 sec past 12-24 hours. I should wear and observe for longer time period for better results. However, it seems doing way better performance prior to service (which was gaining 5 sec a day). In conclusion, it was my first experience with luxury watch brand and Omega (My Rolex watches has not issues so far). I am very satisfied with their customer service and their performance from Omega service center and Omega Boutique at Beverly Hills.
You initially said they did a full service, and if that is what they did, they would have changed parts. Regulation would not require any parts, so not sure what your watch had done...
They always change mainsprings, so parts were changed, but often they are not sent back when it's a warranty service... Just wondering if you had this problem: https://omegaforums.net/threads/a-look-at-some-defective-parts.62631/#post-782726