For every track you select you will need to wait for the download to be completed before it could play. I tried putting in some Mp3 files (128kbps) on my Box account and it took an average of 15s to get each track to play. The loading time will be much longer for lossless.
Also, we will also need to play track by track manually right?
It would depend on your network speed...
I've contacted the developer of Titanium media sync to see if they could add Box support. That would give you seamless file sync over Wifi, always a nice thing to have. Maybe if PowerAmp would support Box playback, then we could really have some fun!