I don't own the BitaxeUltra, but having worked with similar chips on stick and pod miners in the past, it's generally safe to run any voltage, as long as the chips stay cool enough.
In some cases, industrial miners run their chips at substantially higher voltages than the ranges we use on micro-miners, even when using the exact same chips, simply because they have better cooling solutions like high-flow, high-pressure - but very loud - industrial fans, or liquid cooling setups.

Do keep in mind that there's a point where your efficiency starts to decrease, simply because the increased power draw from the higher voltage stops yielding substantial improvements in hashpower, so have a look on that as well.