There is some delay on the first shot, I've tried it just now and I already knew anyway: you can cancel it with your shield if you're fast enough. I also tried to spam it like you said, didn't reach anywhere near this rate of fire: https://www.youtube.com/watch?v=eGe9YGgaIow
But if that's fine with you, then try bringing it to a real game to see how well it fares.
The delay on the first shot didn't bother me while doing that, however, try this: bind your skill on either wheel up or wheel down. It's the fastest way to spam shield aside from macro. Then, shoot gauss while scrolling up or down. As it turns out, it doesn't work all that well (in fact, not at all with my mouse, worse than spamming even though it activates shield way more often). That's because if you're spamming shift this rapidly, suddenly cancelling your gauss shots becomes an issue. You might not even notice that it's cancelling them since it's doing so extremely rapidly, but your rate of fire is going to suffer. Because of that, getting a good firerate with that technique is extremely hard and even potentially impossible.
The guy on the video is using a macro, just read the comments