These cars have driver-selected under or over steer. They're incredibly well balanced. If you go a bit too hard on the throttle coming out of a turn, it'll understeer. If, as your friend did, you lift a tiny bit while cornering, you will oversteer instantly. The chassis has amazing response to throttle steer in a corner. In the corkscrew at Sonoma, where your friend had the oversteer, I can select whether the car under or over steers with the throttle. I am not running V2 arms, but my camber is maxed out given OEM arms, I forget the precise values. The S111 platform doesn't have a rear sway bar, and the front one is tiny, so there's a lot of body roll when cornering (and a lot of grip too!), when you try to fix that roll with stiff suspensions, you actually end up having less predictable handling.
Your friend even commented, that the more throttle you have the more it understeers. That's the nature of this chassis, you're not going to change it too much without some other cost. If you add more rubber up front, you'll change the balance to more oversteer as relative to front, the rear now has less grip. You can't drive it like a nose heavy car. nose heavy cars have great benefit from weight transfer to the rear, but ours is rear-heavy, and the weight transfer under throttle leaves little pressure and little grip in the front. It's a bit like old Porsches, where under throttle, you're sawing at the wheel, while under breaking, it's super-twitchy.