I've had a bunch of them go through here, and I haven't been able to tell any difference in tone. However, if I didn't have any experience with them and was forced to predict what a difference might be, it wouldn't be "a touch warmer / less bright..." Quite the opposite. It seems Maple, which is already kinda brittle, gets slightly more brittle as a result of being roasted. Warmoth has even gone so far as to include a warning in the shipping box about splitting the headstock if pilot holes aren't sized properly for screws. Based on that, I'd expect them to be brighter. But, as I said, that doesn't seem to be the case. Although, Maple is pretty bright to begin with, so maybe it does change, but the change isn't audible. Which is to say, there's no change.
To be fair, how could one tell anyway? Every guitar has different pickups, strings, nuts, bridge, body wood, body design, neck/body attachment... the list is long. And the neck itself can't be magically transformed from roasted to raw with a fingersnap, so you're always comparing two different necks. Even if they came from the same tree and one was roasted while the other was not, you couldn't attribute the difference to roasting because if they were both raw or roasted, they might still sound different.
So, for all intents and purposes, you may as well say there's no difference other than what you might expect statistically from any two different necks of the same design from the same species.