(1) How fast is it supposed to be (well, other than "speed of plot", naturally...)?
In the Voyager episode "Hope and Fear", Janeway's log said Voyager traveled some 300 light-years in an hour before the slipstream crapped out on them, which would make it roughly 15x faster than subspace communication speed as listed in the Trekpedia (which itself is 20 light-years in 53 minutes). I haven't read the few most recent books so I didn't know if they'd ever mentioned more precise numbers that were better, or worse, than those from "Hope and Fear" as the Federation experimented (and hopefully improved) upon an alien technology.
Which leads me to...
(2) Did the Federation make advances in slipstream drive BEFORE Voyager communicated with them from the Delta Quadrant? If Voyager didn't return until, what, 2377, and then they had to figure out how to safely and effectively work quantum slipstream drive the first place, build or adapt a few testbed ships, and then move to a time-consuming prototype Vesta as well as have a handful of production Vestas and other slipstream-equipped ships up and running, that's only a few years between Voyager's return and the events of "Full Circle", which suggests that the Federation has been working on quantum slipstream drive well in advance of Voyager's first communication with Starfleet after the Arturis incident.
Unless...quantum slipstream drive is not necessarily a drive system in and of itself, but rather some kind of bolt-on technology (like the focusing pylon doohickey in front of the Vesta-class main deflector dish) that can be adapted to existing ships. I thought I had read dialogue that Voyager had to be reequipped with new warp nacelles in order to use slipstream and wasn't sure if the Vesta and other ship classes depicted in the Full Circle fleet were engineered from scratch to be able to slipstream without later bolt-on technology, or if it was a fortunate happenstance of their design.
And IIRC from the Ships of the Line calendar, the Meerian-class came out in 2377, right about the same time that Voyager returned. If that's the case, then either the Meerian was built with slipstream in mind from a pre-existing Starfleet propulsion project, or it was an easily-adaptable design to the slipstream tech that Voyager returned from the DQ with. I am inclined to think the former, since it appears that the requirements of slipstream drive suggest that it is the driving influence of the sleek and slender Vesta design in the first place. It is my understanding that larger, wider ships such as the Galaxy and Nebula-classes cannot use slipstream (see below).
(3) Related to #2 somewhat, was it spoken of in Trek Lit if Voyager communicated the technical information and specifications of the quantum slipstream drive to Starfleet between the time of "Hope and Fear" (by which time they had accessed the Hirogen communication network as established in "Message In A Bottle", earlier in the season), thereby giving a few more years of lead time for the program, or if Voyager kept that data to itself until finally arriving at Earth in 2377?
I am unsure if Voyager was able to use the Hirogen network and message back and forth with Starfleet until they began to establish realtime contact via the MIDAS array a few seasons later, or if what occurred in "Message" was a one-off. And I don't know at what point Voyager was able to realtime with Starfleet, but I was thinking it was only a handful of episodes before "Endgame", which wouldn't have given Starfleet R&D much time to work on slipstream otherwise. Additionally, looking at that situation from a security point-of-view, I don't know that I would have entrusted slipstream specs and information to an alien communications network that may have been hackable to the Romulans or other threats. As easily as Voyager accessed the Hirogen network, it stands to reason that other races may have been able to do the same; as tight security as they put on the slipstream drive project to keep it out of the hands of Typhon Pact members, Voyager may not necessarily have entrusted that information to the Hirogen comm network.
(4) Does each ship in the Full Circle fleet generate its own slipstream, or are only Voyager and the three Vestas equipped with slipstream drive and serve as "icebreakers", if you will, for the other fleet ships trailing close behind?
The impression I was left with from "Hope and Fear" was that once Voyager was able to access slipstream, it was able to catch up with the "Dauntless" by following Arturis' slipstream tunneling, in essence "drafting" his ship...and that Voyager's inability to keep the slipstream going for more than another hour was when it ceased "drafting" Arturis and had to turn 180-degrees and begin tunneling on its own.
I didn't know if the rationale behind using the Vestas was because they were the "icebreakers", or if they also needed to have some battlewagons on standby in case the Full Circle exploration fleet ran into some heavy shit. Using two out of seven Vestas in all of Starfleet is a pretty large commitment, especially with the rise of the Typhon Pact occurring at the same time (I believe) as Full Circle.
(5) Quantum slipstream appears to work most effectively when the ship using it is more slender in profile. Does this mean that the wide Galaxy- and Nebula-classes are not able to slipstream at all, or does it just require greater power to allow them to slipstream?
(6) Has it been determined how large the other two ship classes in the Full Circle fleet are? I was of the understanding that one of them is mid-sized (compared to the small Meerians and large Vestas) and another one is hella-big compared to the Vestas. But I didn't know if more specific information regarding those two ships were given later in the series. And, if size determines slipstream efficiency, is the bigger ship just a large cylinder?
(7) Are there illustrations or schematics online of the two aforementioned ships, or of the modified Voyager?