Not exactly sure of the problem with what your formula is. Only thing I do notice is that your first formula is "starts with X" and your wildmatch has X anywhere in it. If you are simply trying to sum volumes field if the value is X* or special volumes if it isn't can't you just use:
If(A <>'x*', sum(volumes), sum([special volumes])
Not sure how elaborate, or frequent, your real use case is, but this is the type of thing I like to handle in the load script so that the user interface isn't slowed down doing calculations that are not dependent on other field values or selections.