One way to upgrade power on a (non-amg) M278 is to retrofit AMG turbos into the M278 engine.
People on MB World report they bolt up directly and just need a tune to work and also routing of liquid cooling. One issue to anticipate is the M157 turbos have coolant hoses, and the M278 turbos just use the oil cooling. Reports are they work pretty well and are still a bit small for the M278 engine as referenced in the build the fastest CLS550 thread and M157 vs M278 thread on mbworld. Of course things like cooler spark plugs and reports of needing the AMG fuel pump are provided by G.
I am not 100% sure you actually need to reroute liquid cooling in every situation. Just like some 157, 278 engines come with split cooling (turbo/engine), some 278s I believe have liquid cooling from MB.
There are several reports, shop reports, and part breakouts that list broken coolant lines, coolant lines, etc.
Often you can find new or like new turbos from AMG cars who have gone to upgraded aftermarket turbos. Which offer more power at the expense of driving less like a NA.
The picture below is from this classified ad. These turbos look really clean from what you can see. Check out these two turbos for sale on MBWorld marketplace with less than 2k miles on them.