One thing w/r/t these volleys that I think could make a big difference in Shroud's game (and everybody's) is to do with footwork, and specifically with timing the split step as he's moving forward.
Gonna go dig up some kind of pro on pro S&V heavy match. Hold on a sec...
That'll do.
Shroud, what I'd like to take a look at, if you're willing, is (A) when Pat and Pete are split stepping, and (B) where they are on court when it's happening. Then look at yourself to judge the same thing.
Those guys are nailing that split step to coincide as well as possible with the opponent's contact with the ball. There's a reason for that.
You (and a lot of amateur S&V mavens) have the idea ingrained that you want to get as far into the net as possible before split stepping. This is not accurate. You want to get as far in as you can BEFORE the opponent makes contact, and THEN split step, even if that's considerably further back. Why? Because at the split step, your body is ideally poised and balanced to react in a coordinated fashion to whatever comes from the other guy's racquet.
If (as with the point in question), you get a little closer to the net, but have to rush to get down to the ball, there are numerous little -- possibly invisible -- things that can be going on with your stroke that can screw up your result. Your weight may not be moving or distributed quite right to feel on balance...your wrist may not be quite firmly enough set...you may look like you're in position, but not be well-balanced enough to block the stroke with any kind of feeling of authority. By getting your feet under you FIRST, you set your whole body up to react as a single athletic unit. Maybe that's a volley you can't make anyway, but maybe it's one you can. And getting your whole body squared away first gives you the best opportunity to allow that to happen.
If you're caught a little further back, and have to drive-step forward, then maybe that seems less that ideal on the surface. But drive-stepping forward from a stable position, even if it's not easy, still puts you in a biomechanically advantageous position to volley the ball back with some authority. Your weight will be moving forward from a stable base, and you'll have time to get your arm, hand, and racquet to the positions and levels of resistance you want.
Pete and Pat? They're very frequently split stepping well before the service line. Does this spell doom? Not at all. Split, recognize, then adapt. Just because you've split stepped doesn't mean you're done moving forward; you're just positioned to move forward (or sideways, or OUT of the way, or even back if necessary) as well as possible.
I'd worry about smaller mechanical details after you get used to the idea of split stepping aggressively and early enough. A lot of the time, little mechanical details will sort themselves out once you get used to getting in proper position before employing them.