I think you can safely remove about half the native bitrate and be perfectly within the range of "indiscernible". I don't like to think of H.265 as a means to use a lower bittrate. I like to use it to retain more quality at sensible rates. 25-30mbps is fine for most content, with a more prized movie getting a bit more overhead. Deleting non-used tracks and the likes is also a plus.