Oh, PICA heat shielding on the Stardust sample mission worked fine at a re-entry velocity of 12.6 km/sec and a heating rate of 1,200 W/cm^2. The Dragon's PICA-X heatshield is already 3 inches think and only about a half-inch burns off during re-entry from orbit, so they have a huge margin. Re-entry from Mars might get into the 3,500 W/cm^2 range, and PICA hasn't been experimentally tested at those heating rates, but it's not predicted to be a serious issue. They would have to conduct such testing, though, and might have to switch to a different material. If need be, they can go up to 10 inches thick with the PICA shield.
Probably the main issue is that they'll want to very accurately model the aero-thermodynamics so they don't waste mass on an unnecessarily thick shield. Getting a heat-shield to survive a re-entry is obviously possible since meteors make even more extreme entries with an accidental shield. Their existence made Robert Goddard suggest an ablator back in the 1920's.
Besides, heat shield design is fun.