I can almost guarantee that any new speed limit short of infinite velocity would be broken almost immediately if the story demands it.
How about you give me an example and I'll try and give you a creative workaround. I don't think there's any situation that couldn't be worked out.
Borg ship came out of a transwarp conduit and has entered Sector 001. The Battle for Earth has begun, and Starfleet is losing ships left and right every minute. The Borg are now within reach of the Sol System. Unfortunately, the
Enterprise is way over at the Neutral Zone at this time. How will our heroes save the day?
That's your scenario.
And any kind of limit is still, well--a limit--and one that will get in the way of dramatic storytelling.
Writing is filled with all sorts of limits. Characters need to behave accordingly. Things need to behave how the writers establish them or possibly even how science establishes them.
See, that's the thing--warp drive
does behave how as the writers have established it. Writers have
always fudged warp speeds and travel times in Trek.
Every Trek series and movie has done it since day one.
As a result, Trek
has maintained a consistency with keeping warp speeds vague and undefined. If anything, what's in error are the offscreen materials that says "warp x equals this" or "the Klingon Empire is x light-years away."
Stories need to have continuity to a significant degree. Apart from that, most series have some sort of writers bible that shouldn't be deviated from. These things aren't hindrances, it's just the sandbox they're allowed to play in. If they couldn't play within those guidelines, then they shouldn't write for it. I'm not saying Star Trek ever had such a guideline, but that there would be absolutely no harm in having one.
For
the most part, Trek has maintained a general continuity. Looking at the whole of it--some 600+ episodes and nearly a dozen films, the continuity violations are actually few and far between. It's just that Trek fans tend to pick these things out and debate them over and over again.
To be fair, though, how warp drive has been depicted in all of the Trek shows have been fairly consistent--ships can cross vast sectors of space in a very short period of time. The only "real" problem is that they have never adhered to the non-canon warp scales or maps of the Star Trek Universe. Even the Voyager broke its speed limit every once in a while when the story demanded it to.
I wouldn't say never. There are several attempts to use those scales, but they are then negated by someone who just doesn't care.
Actually, the only series that tried to adhere to any known warp scale was VOY, and even then they found a way around it. I don't think it is case of someone not caring, but someone knowing that the story comes first.
For instance, going back to Enterprise, they define the time it takes to get to Neptune and back at warp five. Oddly enough, it fits the TOS scale for warp drive almost exactly. The problem then comes when you actually give those figures out and then say that a trip to Qo'noS can be made in four days. Someone is going to do the math, and figure out that the writers screwed up.
I think that someone is going to be in a really small minority of viewers. Even among Trek fans, there are very few of us watching the telly with calculators in hand to see if they got the non-canon warp factors right...
If adhering to either the so-called TOS or TNG warp scales, the ships would be too far away and it would take them months to make those voyages.
I never said that they had to follow the math of what was in the technical manual. All they had to do is lay out a spreadsheet of how long it takes to get from A to B, B to C, and A to C, and then consistently follow
that instead of "the speed of plot".
It doesn't matter what the spreadsheet is or what the math is. It will
still get in the way of dramatic necessity because the speed of plot will always be faster or slower depending on the story. It's the "drawback" of any kind of series whose primary goal is entertainment rather than being 100% scientifically accurate.