Any 1080p50 capable TV will be able to play a 1080p50 HDMI signal from a 1080p50 Blu Ray disc.
So there is no technical reason not to release a high frame rate Hobbit.
"Speeding the film up" is not a relevant technique for digital formats. It is exactly how films used to be transmitted for TV but that was all about aliasing. If the TV station played the film at 24 fps then the 50 field per second cameras would introduce stobing effects, so speeding the projector's motor up and locking it to the field rate made sense.
In the digital domain one has to resample the frames. For 24fps to 25fps a common standard is 3:2 pulldown: "Converting a slower frame rate to a faster one requires duplicating frames or fields. People have been transferring film to video for a long time. For NTSC the film is slowed down to 23.98, and then every other film frame is held for one extra field. (see diagram) This is called 3:2 pulldown. Some of the original film frames now begin on the second field of the video frame. This looks fine while being played, and can easily be removed for conversion to 25 frame PAL or 24 fps for DVD compression. However, if this material is edited without attention to keeping a steady 3:2:3:2:3:2 cadence, a clean frame rate conversion becomes nearly impossible."
This same approach would be used to resample 48 to 50.
The audio is not normally sampled at the same rate as the video and is typically at 48kHz even when the video is at another rate. The audio and the video are separately multiplexed into the MPEG transport stream and kept in sync using timestamps. Therefore there is no need to resample the audio when resampling the video.