Thanks for you suggestion.
Yes, Similarity use multiply threads, in future we add some setting to control threads count in options dialog, in current implementation it use 1 thread for every physical core in thread pool.
About 64bit, maybe in future, with 32bit, we limited by 200 000 sound files ~2Gb, this is very huge amount of files.
About UPnP, network functionality only in experemental stage and not in priority list in current moment.