For 400hp you can use the stock m104 980 engine with just a thicker head gasket. I'm currently running ~480 hp on a bone stock m104 980 motor with 1.0bar boost and a compression ratio (static) of ~9.2-9.3:1 with 2.64 mm head gasket and that's on pump gas .

As to decompressing the setup you are building, I'd do it (apart from custom pistons) using a decompression plate with two head gaskets, that way you wouldn't weaken the pistons.

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190E 3.0-24v (M104 980) turbo @ 0.8 bar
1/4 mile: 2.483 / 13.540 / 175.17 km/h (street tires)