Do you really have no drone even with primary/secondary cats and resonator deleted?
Are C63 stock mufflers different from SL55 stock mufflers?

From my experience, just resonator delete + SL55 mufflers created a lot of drone.
And resonator delete + Supersprint mufflers also created drone.

I really wonder C63 mufflers do not cause any drone because they have different internal structure.

I can tell you that kustom2k1 had all deleted but kept stock h pipe and his sl55 had bad drone
We installed an x pipe
Drone all gone and gave it a nice sound

My opinion sl55 mufflers are the best because they are small so little weight and nice free flowing

I tried that sl55 mufflers... waste of time/money dont do it. Get your hands on some cls55/63, c63 or e63 mufflers and you will be happy.

How was your setup? I'm curious because I have heard 2 with the sl55 mufflers and sounded great

Pri Cats to xpipe to sl55 mufflers sounded poo. See YouTube

The way heard it was straight from lts or shorties with 3" collector Into x pipe and then to sl55 mufflers sounded very nice