I was doing that back in the day when I was lighter for sets of ten with 145.
Probably can still do +135 for a several.
Hollow holds (and eventually hollow rocks) are GREAT progressions after you've got a strong plank. I've gotten to where I can do a 30 second hollow hold while holding a 10kg plate above my head. I tend not to do a lot of accessory core work, but every once in a while I throw these in to great effect.
It seems like guys who actually lift/ train for strength are way more friendly than the other 80% of people lifting weights. Every time i ask the big strong guys how many sets they have left they are very friendly and almost always ask if i want to work in. When i go up to ask the average "amateur bodybuilder" or whatever most of the time i get ignored, maybe a dirty look and its always "I dont know man, a while?" Why is it so hard to be polite to people at the gym?