I am a Mercedes guy who has a few turbo setups under his belt and I just wanted to add a few points after reading several pages of this fantastic thread. I'm not trying to step on any toes and I haven't read it all so bear with me:

The MAF maxxes out at just over 5.1 volts on a OBD-II logger at around 1-3psi on the M11x series engines. I am surprised you have been able to make the car run as smoothly as it apparently does with it being a blow thru setup (I read you still have it in front of the TB). I would strongly recommend you move the MAF in front of the supercharger and clamp the MAF at 4.95 volts and it will make your tuning so much easier, or remove the MAF connection altogether due to it's sensitivity. It won't stumble in part-throttle boosting and and it simplifies the entire construction of the fuel map. You can literally take one number from 14.7 psig down and set it at that. In my E320 with 750cc injectors, from 14.7 psig down(effectively the closed loop range), was -71.3 then at 15.0 psig and up began the real fuel map.

In terms of power holding don't set that engine at 14 psi. The pistons are iron-coated, cast silicon-aluminum pistons. Although on paper they are made of the same material as the cast pistons in the 32 AMG and SRT-6 have, they are cast with less pressure. Even though the reset of the bottom end is forged, a few good detonations and the ring-lands will go bye-bye on the pistons. With a 550hp capable S/C you'd be safe at 12psi max pulling several degrees of spark timing.

As to the issue of belt slippage. Couldn't it just be the stock belt slipping over the accessory side of your twin idler pulley? It has hardly any belt contact to begin with and they slip badly with as much as a dirty look at the bearing.

On the issue of spark timing. It sucks....the F/IC that is, and the F/IC-6 specifically. The F/IC-8 is AWESOME. Zero problems with the Mercedes crank trigger on a rear turbo CLK430 I built and it has the same trigger as the M112. The F/IC-6 will interfere like hell and even with the resistor mod on there you have to put one on so heavy it will shut the car down at idle and that's sure as hell not worth it.

Here's that 4.3L M113 CLK with a draw thru(yes the MAF harness was extended 10 feet) and the F/IC-8. No high rpm miss or stumble. The MAF was left untouch other than to extend the OE harness(you can see my test harness in the first video).
FIC with the cam sensor hooked up backwards so it was batch firing

With 70mm turbo

With 60mm turbo