We have little official/canonical data on how fast Warp drive is on an actual warp scale.
What we do know however is that most of the time in TNG, Warp 9.6 was a temporarily sustained maximum velocity for the Enterprise-D in early Season 1 of TNG.
It's sustainable warp velocity was at the time Warp 9.2... past 9.3 (at least according to the pilot episode, the ship is past the red line and will likely be able to sustain that/any further increases in velocity for a limited period of time).
We also know that Data said in 'Where no one has gone before' that it would take the Ent-D over 300 years to traverse 2.7 million lightyears at maximum warp... indicating a speed of about 9000 times LS.
Now, what that maximum speed is, we don't know for certain, however, if Paris' statement that 9.9 = 4 billion miles per second... then Ent-D maximum speed would have been LOWER than Warp 9.9 (probably around Warp 9.6 given that for most of TNG the Galaxy class topped out at that speed).
Bear in mind that most Federation (and other ships) in the 24th century were not really seen traveling or being able to sustain Warp 9.9.
Yes, Voyager was stated it's top cruising speed is 9.975, but in the episode 'Threshold', the ship neared 'maximum warp velocity' upon achieving Warp 9.9 and the computer said 'structural collapse imminent'.
This means to me that the writers were either wrong... or, Voyager's top cruising speed is technically Warp 9.975, but due to the pull to the DQ (which was violent), the ship suffered catastrophic damage to it's hull and engines (which we witnessed in the pilot episode, and the Warp core had a microfracture too which was later repaired), effectively reducing it's maximum sustainable cruise velocity from 9.975 to below 9.9 (or in line with what we saw in TNG).
Ergo, from canonical point of view, Paris' statement seems more accurate because no ship was usually seen to have been traveling at 9.9 (it was usually well below that).
Besides, the closer you get to 9.9, speeds start increasing by quite a bit, and past 9.9, every incremental increase would result in exponential increase.
So, I'd say that the Enterprise-D Warp 9.6 = 9000 times LS (which it would be able to sustain say indefinitely/or for as much fuel the ship has later in the show as the engines were improved/modified but not during Season 1).
Voyager's cruising speed of 9.975 would bring the ship back to the Federation in just under 7 days using exponential increase with every increment past 9.9, ... but, because the ship suffered massive damage in transition to the DQ, it was effectively stuck with using much lower speeds and had to conserve energy due to variety of problems as they happened - plus, Voyager was seen cruising at Warp 6 for the most part - seems like a really low speed when its official specs put its top cruising speed at 9.975 - and 'top cruising speed' means that your engines can run this speed continuously with minimal maintenance, or for as how much energy you have to run the engines - higher warp speeds than that would forcibly shut down the drive much faster).
Also, of note would be that the USS Prometheus (from Voyager's 4th season) was seen using Warp 9.9 in a sustainable capacity.
So, its more likely that the writers might have meant that Voyager's top cruising speed was Warp 9.75.
Anyway... it seems more likely (at least from a canonical point of view) that Paris statement has merit because most ships of that era were never seen or said to be using Warp 9.9.
9.5 and 9.6 yes in emergencies, but anything more and you risk structural collapse (at least until just prior to and during/after the Dominion War - because Warp 9.9 would seem plausible by that point with SF's desire to improve their warp engines).
Also, in their decoded message from SF, it was said by Admiral Hayes that 2 deep space vessels were redirected to Voyager and that they should meet up with them in 5 to 6 years... indicating these ships might have had better Warp drives (capable of achieving and sustaining say 9000 times LS) than the rest of the fleet as they'd be needed for deep-space exploration.
