If you want constant acceleration, you need to maintain constant force. A higher-velocity vehicle applies that force across a greater distance per unit time, and therefore requires more power.
Accelerating 1 metric ton at 1g requires 9800 N. So at peak velocity (18 km/s), that's 180 megawatts per ton, which is more than 700 times the power to mass ratio of a Tesla P100D.
So, there's surely a safe speed limit built into the materials being used? It's probably faster than the speed they're starting out with, but I kinda assume it can't be fast enough to shorten the trip to hours from days.
As an aside, moving people is probably not the most profitable use of the thing for the foreseeable future, as cool as that sounds. Getting a satellite into orbit for a tenth the cost is revolutionary (though they mention the cost compared to the shuttle, which is much more expensive than rocket transport...so they may be cherry-picking numbers to make this seem more revolutionary than it is).
Anyway, I can't think of how they could accelerate much beyond the fastest non-maglev train speeds without tearing apart the cable and car, but I may be underestimating the strength of carbon nanotubes, and whatever other component materials they're using. Perhaps there's a materials nerd here who knows.