I have successfully run my BOSE BT speakers through a floor and a window, but walls definitely affect performance.
My M2P Pi3 server is in the upstairs study on the window ledge (north facing so no sun concerns)
The BOSE is normally below the study in the kitchen, but also reaches out into our back garden. Our garden is a reasonable size but it will cope line of sight all the way to the back wall.
It doesn’t like being moved, and garbles the sound, but stabilises as soon as the speaker is stationary.
As for synchronisation, I haven’t seen any problems. I have a Pi3/HifiBerry/WIFI/ PiCorePlayer in the lounge walking between the lounge and kitchen I can’t detect any sync problems.