The whole issue of high vs. low protein would make your eyes sping.
I can only speak from my own experience, and even then its not conclusive evidence that a high protein diet is bad for the dog.
I had my boys on high protein: First Orijen, then Horizon Legacy.
Bacca was having other issues, including being diagnosed with hypo-thyroid.
For liver, the tests you want to focus on are ALT/ALS and bile acid.
In the course of various blood tests, we found Bacca's ALT scores had soared from 289 to 641 (in 1.5 months) (normal range is 5 - 95) and his Byle Acids were also high. Took him off high protein (now 24%) and had him retested 4 months later. His ALT was down to 203, and his bile acids had also lowered.
His is due for more bloodwork, to see if the combination of reduced protein along with a supplement (Hepato-Support) has stabilized his liver function adequately and if his counts have reduced further. If so, then it is less likely that the problem was a liver shunt, however still not to be ruled out. But at least it would indicate that it could be managed.
The alternative diagnosis is the elevated levels were due to the high protein diet. Or maybe that a high protein diet is not favourable for dogs with the risk of liver and/or kidney problems.
Unfortunately, when I was researching high vs. low protein - for every article I could find in favour, I could find as many against. So if you are inclined one way or the other, you'll find lots of justification for your position.
I know this is not much help. I guess I would sum my experience up with "with everything, moderation"!