Mike Hedlesky used to post here, he has pulled over 800 using hook grip. I think his best pull was around 850.
I don’t really see any advantage of using a mixed grip, if I was going to coach a beginner I would get them to pull with a hook grip. It’s harder to get used to once you are stronger since the weights you will be lifting will hurt your thumbs a lot. There are plenty of lifters out there who only switched to hook grip after a bicep tear, but it looks like it didn’t work out for all of them. Chris Duffin is an example, he can pull 1000 with straps (on a DL bar with thick plates…) but bombed out of his last meet due to grip while trying to pull what, 750?
On high rep sets. Last year I did a hypertrophy phase and was doing sets of 8 on deadlifts, I started using straps because it seriously felt like my thumbs were going to burst. I have never had grip issues pulling heavy singles or maxes, but doing more than 3 or so hard reps can be an issue because of my hands sweating. If there is a drawback to hook grip it is the fact that doing reps is harder, but that’s why straps were invented.
People are pulling big weights with mixed grip still, as long as they don’t tear their biceps there is nothing to stop them. Steve Johnson pulled 900-something at Boss of Bosses. Krzysztof Wierzbicki pulled about 940 at the World Games in a single ply suit and has pulled close to 900 raw, and that was all on a stiff bar and drug tested. Give Wierzbicki a deadlift bar and unlimited PED use and he might beat Belkin and Woolam.