Has anyone tried this, or know if it will work? I tried to do some research but i haven't found anything on the forums.

It looks like the exhaust ports on the 113k heads are a little over 3mm larger. I understand its a ton of work for such a little gain, but i'm thinking if you do cams at the same time it may be worth it.

You will lose a little compression. The chamber on the K heads is 2-3cc bigger.

I wouldn't - you'll LOSE power unless you add the supercharger or some other form of forced induction. I'm not sure the cams are any different.

The M113k engine from what I recall also has beefier internals (forged pistons/rods, etc), so there's that too...

just go all the way to m113k engine, with msc headers you will be making ton of power. i have kleemann cams that i'm selling if you want even more

i kept my car NA and did everything that can be possibly done without going too crazy and i'm just around 375hp to the wheels(p&p heads, enlarged TB and intake manifold inlets, 2.5 dual exhaust with msc headers, more aggressive cams from w203/w209 amg, and some other tiny changes). with m113k engine in the w208 you will be putting down closer to 400hp with a fraction of cost and better all around engine. just my advice, that seems to be the best mod to do for our cars.

You will lose HP, it’s a downgrade. The NA heads are better heads

if i have the NA heads should I port and polish those and swap them on to my M113k then?

It is true the NA heads are more desirable for performance.The M113k cams are specifically machined to each head as well. It is not recommended to swap cams on the K heads without some careful measurements.
The NA heads flow better on the intake side according to some of the W211 folks who get wild with mods.
The K engine has lighter pistons heavier rods and custom flywheel and balancer.. The K heads will bolt on and have no EGR port on the passenger head.
Hope this helps a bit any feedback welcome.
Good luck with your build.
Best, Gator

can the ml55 heads / cams & injectors be swapped over to ml500 ? I have a ml55 in a junk yard near me so nows the time cause I also need to replace the head gaskets on my 2003 ml500 because my girlfriend drive it without the overflow cap on until it overheated 🥵. So while I have the opportunity to rebuild my m113 motor I would like to upgrade the performance possibly with the ml55 camshafts or even the complete heads ! . Good news that leads me to believe I can do this swap is on FCP euro site it says the same HG kit for my 2003 ml500 will also fit the 2000 ml55 . My next question is would I need to reprogram the ecu on my ml500 if I perform this swap ? I assume yes . Thanks 🙏