I've seen appalling driving in both a sheddy old Fiesta with chav cans and in a 458 Challenge car on track days. I make zero assumptions as to the level of driving I expect from the guy behind/in front, as any one with a big enough chequebook can buy anything they want, regardless of talent
I'd consider myself an experienced track day driver, but I'm not sure I'd be happy with overtaking on either side. I've been on days where it's been allowed in parts (Rockingham is the one that sticks out in my mind), and tbh it just encourages idiots to do idiotic things. I'm sure it does work, as lots of companies do it now, but each to their own on the comfort for that.
Worst track day I ever did was at Anglesey with Circuit Days years ago, we had GT-Rs and Evos racing each other and undertaking through the the last corner by the pit wall (the single most dangerous place on the whole track!) but I went back with them last year and they had massively improved and it was one of the best run days I've ever done. I'd use them happily again.