I wonder if the Dragon V2 heat shield could withstand the extreme speed of a reentry from the moon. Or is it only capable of LEO flights?
Yes. According to SpaceX; PICA-X, which is SpaceX's proprietary derivative of the PICA (Phenolic Impregnated Carbon Ablator) heatshield designed by NASA, can withstand reentries from "Lunar and Martian Velocities". Martian velocities being even higher, of course. Source from SpaceX here.
PICA itself was used on the Stardust spacecraft which reentered at a speed of 46,500km/h. Apollo craft generally reentered at a speed of ~40,000km/h. An Earth reentry from Mars would, depending on how much energy you choose to expend, be between 49,000km/h-77,000km/h. Wired article.
Another source is testimony Garret Reisman offered, on page 4, where he says:
Designed in partnership with NASA and fabricated by SpaceX, Crew Dragon’s heat shield is made of PICA-X, a high-performance improvement on NASA’s original phenolic impregnated carbon ablator (PICA). PICA-X is designed to withstand heat rates from a lunar return mission, which far exceed the requirements for a low Earth orbit mission.
If he said it in testimony, it must be true, right? The proof as always is in the pudding, so lets see them do it, but they think they can.