Seeing as blu-ray maximum bitrate is 40mbps I don't think streaming on 100mbit network will be a problem.
Not so.
In order to perform the standard BluRay functions, every BD drive is capable of at least 2X read performance. The drive also supports burst reads from its track cache at much higher rates than that.
This is part of what allows fast forward (although skipping occurs) and other trick play options.
2x means at least 80 Mbps and usage at 80% on an Ethernet style network tends to have issues with consumer switches and routers. Packets are lost and must be resent.
In addition, there is almost none of the burst capacity that you would have even with the lamest USB2 interface (which is in practice three times as fast as the ideal 100 Mbps of the Ethernet).
And, as lampshader mentioned, there might be other traffic sharing the network segment.
A streaming server could certainly play HD on a 100 Mbps interface. A XBox 360 does so easily. A streaming server, however, uses strategies appropriate to a network connection. By contrast, a BluRay player, even when playing back from a network image, uses the strategies and guarantees of a local BD drive on a much faster interface.
That difference tends to mean that playback will most likely be unsatisfactory on such an anemic transport.
To compensate for the difference between local and network playback (without redesigning the player) you need a lot of headroom on the interface.
Gigabit provides that headroom and is very low cost or free with most devices. And no, you don't need to replace your existing CAT5 wiring to use it. It was designed for CAT5.