If this is something that worries you, feed another food. There's tons of other lower ash options out there. A lot of companies won't publish or disclose their ash %, so it's hard to compare, but 8% really isn't high, let alone "dangerously high". I've also heard that high protein can cause kidney issues too, but until I see scientific proof of this, it's just internet hearsay.
Feed what you feel comfortable feeding. There are plenty of people who feed earthborn with good results.