Actually, we know for sure that pronation adds to the swing speed, right?
So if you're hitting topspin second serves, with the exact same grip as you use for a flat first serve, won't you use LESS pronation, since you're trying to spin the serve by glancing at the top right of the ball? So you're holding back the pronation, which leads to less rackethead speed.
By using a slight twist of the grip from continental to just a little eBackhand, you can use your flat serve pronation swing, and hit a topspin serve using the pronation and added rackethead speed.
Lots, I'd say MOST, ATP pros adjut their grip slightly from flat first serves to heavy topspin second serves.
You can try it shadow swinging and LISTENING to the sound of your racket.