The -V will have extended norms at some point. Pearson has been projecting extended norms coming out some time this year, but no word yet on a more specific timeline.
For a very mathy kid, I would give the edge to the -V over the -IV, as one of the new fluid reasoning subtests has pretty good associations with quantitative reasoning (especially of the algebraic kind). The WISC-IV PRI is very much a mixed measure, with one visual-spatial task, one nonverbal fluid reasoning, and one fluid reasoning with a fair amount of access to verbal mediation. Since she's strong in visual spatial, it may be that the WISC-IV PRI would also capture her strengths, at least in terms of numbers, even with its mixed nature. As a rule of thumb, though, it's still preferred to use the most recent norms (and the -V has now been out for over a year and half). As long as the examiner follows standardized administration procedures, a WISC-V administration given now will have the option of re-interpretation with the extended norms in the future.
