QF lad, i flew MS from Sharm-El-Sheikh to Cairo and back in Y towards the end of last year. From what I could see of the J cabin, there really was very little difference, seating wise probably similar to the dom/biz setup on QF's 734's.
Cant comment on food, but the Y 'snack' consisted of an unidentifiable pastry/roll and a carton of joice both ways.
Cairo airport is indeed a pit. I didn't notice any lounges in SSH or CAI although Biz definitely has seperate check-ins, that being said though check-in both ways was empty and took all of 30 seconds - The internal flights in CAI have their own seperate terminal to the Intl flights.
If you feel the $20 offers value for money for a slightly bigger seat on a circa 40min flight then go for it, but $20 goes a long way in Egypt....
Edited to note: My experience was far from stressful, and i doubt it will be as bad as you think. Just watch out for the luggage porters who will demand a tip after offering to help out and the dodgy taxi drivers who have a cheap taxi parked on the other side of the airport. If you want a taxi I suggest you ask one of the security guards - Look for the guys in White with the occasional AK47 slung round their necks!