It probably will pass H.264 video through and convert audio to AAC and possibly to AC3 as well. Most BDrips come with DTS or Dolby TrueHD so they must be downconverted to AC3.
Skipping video conversion is huge timesaver.
Mac is more capable than AppleTV. So it can play also High Profile 5.0 videos, that AppleTV can't. Have you checked what H.264 profiles do the movies have?