Hello,
I think I maybe had a smilar thing with my 435d. Now it's running 407hp :-)
When I first got it tuned I got 375hp which was annoyed me a little. Ecotune mentioned at the time they had to take it down a step for whatever reason. Anyway I knew from my last 330d that carbon build up can be bad so I had a look at mine....
https://photos.app.goo.gl/cRLAJq8JY9B9MQhC9
The photos say it all. I did a wallnut blast and cleaned the manifold out with sodium hydroxide.
I then fitted a new ebay wagner copy, performance Air filter and deleted the swirl flaps.
Took it back to get tuned and they said the DPF was clogged so they did a manual forced regen and remapped again under the instructions to get as much out of it as they could within safe limits and I got 407hp with 730nm.
There is a really cool app for out cars now called btool expert (andriod) . Suggest you buy it with a carista Bluetooth adapter (15 quid on amazon). The app will show you some important stuff. Mainly what you want to check is:
1. EGT - should not really see over 350c normal driving, 550c if your booting it hard and red lining. About 600c if its doing a regen.
2.Target boost vs actual boot (this will tell you if there is a boost leak), it's measured in hpa and remember to take away atmosphere pressure (1000hpa). Stage 1 you should see about 3200hpa, stage 1+ 3400hpa
3. Dpf back pressure test. If its outside limits the ecu will start to pull power as I understand
4. Fuel pressure. Our cars should run 2000bar at WOT/full boost as I understand
I'm no expert but that been my experience so far!
I had the DPF off for a while and it's not worth it for the Mot fail/invalid insurance and whistle noise that's really loud!
You can also request a regen with that app. I recently did 5 in a row and got the back pressure lower than I have ever seen. I have an issue with my DPF just now not getting hot enough which I think might be down to a glow plug fault.
Hope this helps.
Cheers