sure the MM can do what you want, though i think your concerns are not specific; if your phone can load a website fast, the G3220 can do it much faster. you can always turn off flash in chrome, safari turns off flash by default now... download speed (P2P or not) is more dependent on your network setup (gigabit ethernet for LAN and n/ac for WIFI). if your definition of file server is just storing files then the MM will be short on space, but if you are asking about how powerful the media decoding is then the MM is more than capable.
anyway, if your real question is whether you should try out OSX, i don't think this is the right subforum to ask

you should direct your question to the AV/HTPC subforum.