Best body shop for high-end cars in the Austin area is Berli's in Pflugerville. Their work is impeccable, but they cost more than most shops.
Since Penske now owns the Round Rock Auto Center and all its associated dealerships, Round Rock Collision Center on Mays Street just north of Old Settler's Blvd. is the preferred body shop for BMW of Austin (also owned by Penske). I had some paintwork done there for my E92 and it was expertly done at a reasonable cost.
2013 F30 335i M-Sport 8AT/MPE/MPPK, 2011 128i M-sport Cabrio 6MT/PE