http://forum.feed-the-beast.com/threads/steam-limitations-in-liquiducts.16101/
The second post in that thread said something interesting regarding liquiduct limits. There does seem to be an upper limit, but it's (apparently) at 52 steam engines worth (something like 2k mB/t). Though this may be quite a high number, it is still a limit to be aware of. A 36HP boiler is already 720 steam/t, so according to this information you will surpass the max of a liquiduct at 3 boilers.
I see that this was posted back in march. I believe King Lemming had posted something recently in the last couple months that stated that liquiducts and conduits were now infinite/lossless. I think it had something to do with the way he was stream-lining his code and removing the per conduit/liquiduct calculation as it was causing lag in large setups and his #1 goal for his mod was reducing lag. I think it now only checks that there is a pipe connection with the right liquid in it, and he does all his calculations on the connections to the machines instead of in the pipe.
Edit: I could be entirely wrong though...