Impedance matching is often necessary in various circuits, and depending on how sensitive the Variax electronics are, a change in the impedance match will affect its performance I'd imagine.
I'd thought about trying an Alumitone Deathbucker, but at 5.2K it's probably on the "thin" side, and those pickups are already trebly. 8K-ohm is pretty bog standard for "PAF" style pickups, and it will be fairly trebly as well. More winds = higher DCR and inductance, which decreases treble response.
Since the spec is 6-8k, the nominal is probably 7k, which is what a hotter strat-style single coil can use, or a "PAF" style humbucker. Even a mini humbucker can work. Tons of options.
This raises another question: has anyone tried wiring a resistor between hot and ground leads of a higher DCR pickup to mimic something within spec? It would load the pickup, which would decrease volume, and possibly treble response, but would be interesting to note the differences.