Is there a functional difference between > *:first-child and > :first-child?

They are identical even if we consider performance. From the specification we can read

If a universal selector represented by * (i.e. without a namespace prefix) is not the only component of a sequence of simple selectors selectors or is immediately followed by a pseudo-element, then the * may be omitted and the universal selector's presence implied.

So writing > :first-child should mean the same as > *:first-child for the browser.

You can also read

Note: it is recommended that the * not be omitted, because it decreases the potential confusion between, for example, div :first-child and div:first-child. Here, div *:first-child is more readable.

So it's not only a matter of preference but it helps avoid confusion and make the code more readable.

In the new sepcification we can also read:

Unless an element is featureless, the presence of a universal selector has no effect on whether the element matches the selector.

and

Note: In some cases, adding a universal selector can make a selector easier to read, even though it has no effect on the matching behavior. For example, div :first-child and div:first-child are somewhat difficult to tell apart at a quick glance, but writing the former as div *:first-child makes the difference obvious.