How much latency should I expect in an Instreamer/Exstreamer setup using BTRP ?

We generally say "expect 500ms", it very much depends on the codec used, network bandwith - and very important, network jitter !

NOTE: the higher the encoding quality and bitrate, the better (lower) latency .. this sounds awkward but due to buffers being "shorter" in that case, it's true.

Best case about 200ms, if the network adds a lot, or you have to configure higher buffering (because of network jitter), then numbers are higher of course.