As @nwallace mentioned but that various things to factor in, but to be it bluntly, I don’t think the spindle upgrade will do much (if anything) regarding increasing max achievable DOC. A spindle (and a water-cooled one at that) is a great way to have a quieter machine, and in some cases a lower runout for detailed work with very small tools. But DOC, that is pretty much bounded by the machine’s rigidity limits and the rigidity of the workholding.
There is a fantastic thread by @LiamN about deflection (among other things), and while the Shapeoko3/4 is quite a capable machine, at some point its ridigity at the spindle level will be limited by the v-wheels. Hence why the Shapeoko Pro with its linear rails improves on that aspect, and allows for greater DOC. Larger/heavier industrial machines that weigh a ton will be even more rigid and allow even greater DOC.
On a Shapeoko, you don’t necessarily want to max out DOC anyway, it has been proven that it’s more efficient (in terms of material removal rate) to use a moderate DOC but more aggressive chipload (RPM & feedrate). To feed faster, while maintaining a constant chipload, you need to also increase the RPM. And that’s where the spindle shines: the noise of a spindle at max RPM (usually 24.000RPM) is much more bearable than a trim router screaming at the same RPM. So once you upgrade to a spindle, you tend to use the higher end of the RPM range much more often than when you have a router, or at least I did. And that enables/requires faster feedrates, so higher material removal rate (at the same DOC as before)