With hybrid turbo, 4bar map, downpipe without cat + dpf, stiffer valve springs , bigger cooler, transmission upgrade and Quqife.
Gives possibility to get over 400hp/1000nm.
OM642 is not so good engine, inline 6cyl is better.
The turbo might be ok.
It is the bad downpipe, that causes this situation, air (exhaust) do not flow properly.
That's why boost pressure do not rice - one reason.
The other is, that vanes are too open, after you have done your "things", the remapper have to do his.
If egr is clogged, remove the blockage, the ecu needs exhaust pressure information to make the turbo work properly.
Secondly, there is about 50mm throttle in the beginning of downpipe, chance the whole thing to 3" one.
Thirdly, you need a good cooler, that is something very different, what is...
Only different 250/220 cdi is that 250 have bigger injectors.
Quite easily 220cdi => ~500nm/230hp
Personally, I have a coupe 335d too, in fact two. The other one is true M-sport, the other is racer.
M550d has three or now four turbo's.
I have had a quite nice results for them.
Sorry to say...
If the 270cdi turbo is vacuum controlled, change is like plug and play.
Otherwise => little work.
If you are willing to get something more for the bigger turbo, you need a remapper, likely a good one.
You don't need to do anything for the exhaust as far as you use orig injectors.
Egr and DPF removed and softw.
No other mods
Glk OM651 220cdi is equal to 250cdi.
There is a possibility have even more easily, but fuel system do not support +300 power as stock.