I have been reading through this thread with much interest as I have similar issues with my E91 330D (MY2010).
Slow warm-up, hardly ever getting into the 80s C, and a good 15% thirstier than it was a year or 2 ago (now 64 k miles, full BMW service history and extended warranty).
Catalyst temperature only occasionally getting above 350C (more normally in the low 200s).
Dealer detected errors on 5 of 6 glow plugs, and GP control unit, replaced these, and after some pressure from me also replaced the main coolant thermostat.
The improvement was significant, but very limited timewise.
First drive home the fuel consumption got back to 42mpg (UK gallons), even in traffic (which is how it used to be), but has since rapidly declined to about 32mpg.
Monitoring various parameters with the BMW Drive Analyser app, shows strange (to me) Lambda numbers, which constantly return to 1.40 once warm, with little variation as the accelerator is modulated.
The only time the Lambda gets throttle sensitive is when the engine seems to get into a regen process (holding lower gears for much longer, cat temperature then climbs up to high 500s for a few moments). At this time the Lambda changes just as I would expect it to.
EGR valve has not been replaced.
I am tempted to change this myself, but do not want to invalidate the warranty.
Please see attached link, where I go into more details.
https://www.pistonheads.com/gassing/...f=72&t=2068053