Been reading this thread with some interest. A long time ago I might have said different stuff, but like Torstan, I also read Iain M Banks books and he has a masterful fixed system in place that deals with travel and time in space. He has FTL because basically you just have to have it given that its 4 yrs to get from Sun to Proxima Centauri our very next closest star. A campaign might burn through your character lifetime in a few system jumps without cryo or something equally as fantastic tech.
In Banks' books the Culture have various grade ships from fairly light transporters to these things called General Systems Vehicles (GSVs) which are vast billion population scale floating islands of ship. These things go the fastest but at the same time they dont deviate course or stop much either so you normally need to hop onto a ship and accelerate up to very fast speed and dock with it to hitch that ride which is like that FTL barge. So maybe any ship can do FTL but not for very long.
He also seems to have some heuristics about how long the comms is to and from a GSV. Now obviously you need FTL comms or else you cant ask to dock with something going FTL. I reckon its important that the comms is a lot faster than the fastest ship but not infinite speed either.
Another slightly unrelated point is the compute power which in his books the best is in the Minds of the GSVs. All smaller droids have varying degrees which although high is still on a measurable level so you can for instance, comms to the closest GSV and get a better answer tho it might take a while for the result to get there and back.
So yeah, read all his Culture novels. Got the Algebraist next to me unread at this point tho. Something to look forward to I remember Player of Games being one which had a lot of this time delay, ship speed stuff in it.