Don't ever set foot into that so called reputable shop ever again.
Reducing backpressure won't decrease power, period. Sometimes it can affect low end torque by shifting the torque peak to the right a bit but it won't reduce peak power.
To answer your question on how much power to expect, the answer is very little, expect 1-3 HP max from an axle back exhaust. Power gains from exhausts on the M3 (and most cars for that matter) are made by removing the cats or adding highflow cats. Axle back exhausts are for sound and not performance.
If you want a nice sound put on a muffler. If you want a performance gain add an x-pipe with high flow cats or even better, no cats at all.
2011 E90 M3 ZCP - Individual Moonstone/Individual Amarone Extended/Individual Piano Black With Inlay:LINK!!!
1994 Euro E36 M3 Sedan - Daytona Violet/Mulberry:LINK!!!