I'm interested in squeezing a bit more horsepower out of my non p31 C63, but I do not want to compromise on reliability.
What horsepower can you safely add before you need to start upgrading the transmissions and the internals? I have the 2012+ MCT transmission

575whp

If you help me find Tracer Tong I can tell you.

Tracer Tong's laboratory is located in Hong Kong, just beneath the Luminous Path compound.

The path of enlightenment is closed to gwailo! 🤣🤣

It really depends on how far you want to push the envelope. I would do preventative maintenance before any mods. Make sure your car does not suffer from the headbolt issue (pre 2011). If it does, then replace the headbolts with ARP head studs from Weistec and might as well do the headgasket while your in there.. After that You can go with aftermarket headers (do not go ebay route), ROW air intake with AFE pro Dry S filters, and a dyno tune. The car is pretty dialed in from the factory. You should be around 440 whp or so wit just headers, intake, and a damm good tune.

The breaking point on our engines differs from engine to engine. I've seen some blow at 600 whp and others at 800whp. The transmission as its tuned can withstand up to 630whp before experiencing slipping.

If you just want a teeny bit of power, I would do a tune and if you want more, headers. To get the max horsepower without genuinely compromising reliability is to get a Weistec stage 1 supercharger. Doesn't need any other modifications and is also CARB legal. Adding a blower is basically the only way to get the maximum horsepower out of our engines. Keep in mind adding more power than originally intended will compromise reliability in some sort (additional stress to engine) but our engines are pretty solid and can take a beating.

Cool, that is good news thank you.
I was aiming for no more than 600hp at the crank. I usually daily the car after all.
So other than a mild tune, headers, and ROW boxes and filters, I shouldn't need to upgrade anything else? No need for forged pistons or tranny upgrades?

No, but expect shy of 600.
Closer to 550-575 crank I would guestimate

If you add Magic Intake Spacers and throw in a bottle of Boostane, 600whp is totally possible NA.

True story, I read it on MBWorld.