I made some transcoding experiments with dBPoweramp some time ago. If my memory isn't failing on me (which is quite likely lately) somehow the thing manages to fit 24bits from a lossless file into mp3. Don't ask me how, nor if the result had indeed any extra info resulting from the 24 bits source. The file was bigger than the one made previously processing to down into 16 bits. I can't recall if I tried making V0 and 320 or only 320 files.
In any case I see no point at all in using a "higher information" source for the 24 bits, if you're fitting them into a lossy format which is discarding information to begin with.
I could consider purchasing Hi-res lossless files from Apple if price is more or less sensible in the HDTracks range, but not at a silly high price. Not that I see much benefit to those 24 bits if they don't come with some extra sampling rate. Sound-wise 24/48 doesn't make much of a difference (and depending on the systems for worse) compared to 16/44.