Your max theoretical would be 50MB/s on that connection, so 25MB is not terrible. I think there is some notable overhead on steam (check summing or encryption) as with my 1gbps connection previously I would never get past 100MB/s (800mbps).
Speeds seem to vary based on the game and based on releases, interestingly. Some larger new-releases (Baldur's gate) were only hitting <100MB/s. I imagine it comes down to how loaded up the local steam POP is and maybe even the specific server hosting that game's files.
Aside from that release, almost everything that is a good size (>10GB) has gotten north of 100MB/s pretty regularly.