Christopher, Arturis' slipstream drive may have had multiple speeds, but there's no evidence from what you or David Mack has written which implies Starfleet's do.
I don't see the logic of that assumption. Starfleet's slipstream drive is directly based on the technology of the
Dauntless. It's an improvement on that technology in a number of ways. How could it have
less functionality?
And there's certainly nothing I, Dave, or Kirsten has written which implies that Starfleet slipstream
can't achieve multiple velocities. Granted,
Unworthy does use the phrase "slipstream velocity," but it's ambiguous; two of the specified distance/time relations (3000 ly in 8 minutes on p. 31 and over 20,000 ly in 60 minutes on p. 92) correspond to roughly the same speed, but a third reference is more ambiguous; p. 73-76 refers to a 20,000-ly flight that began 5 hours before and ended an unspecified time in between. Maybe there is a default, preferred slipstream speed, but that's hardly evidence that it's
impossible to achieve any other, and I still see no grounds for even conjecturing such a bizarre notion.
Anyway, the theory of slipstream I described in
Greater Than the Sum certainly contains within it the inherent assumption that slipstream technology is capable of achieving any desired speed. The drive uses quantum manipulation to modify the curvature of spacetime, allowing that modification to be done with far more precision and control than warp drive is capable of. That enables it to create spacetime metrics that allow for far higher speeds. Logically, it follows that greater control and precision would translate to a
greater ability to adjust speed, not a complete absence of such capability.
Remember that the slipstream drive was his race's method of FTL travel, just as warp was to Voyager. Maybe when Starfleet is able to refine the technology further and benamite crystals are able to be found/synthesised, Starfleet will get faster slipstream.
As discussed earlier, Starfleet's slipstream is already immensely faster than what Arturis's slipstream was reportedly capable of. "Hope and Fear" said the drive could cover 60,000 ly in 3 months; "Timeless" showed 10,000 ly covered in what appeared to be only a minute or two of screen time.
Unworthy seems to suggest a standard slipstream speed of 350-400 ly/min, or more than 250 times faster than Arturis's drive.