I'm considering a similar setup. I'll be boxing 1 war and 3 mages and may have a friend healing as pally if I don't box the healer.

Apparently spell cleave is superior exp/hr if you can manage some ridiculous pulls and blizzard/Nova kite. These pulls look really hard playing 1 char and I've tried boxing them with 3 mages and had a rough time. That's not to say it's not possible but will probably require practice and setup for every one of those crazy pulls. So I agree with you on this point unless you have some 1337 skillz.

I tested 4 warriors+1 priest on the stress test with IWT. TBH it feels clunky with IWT slaves sometimes running off. It's do-able but I found 1 warrior, 3 mage, and 1 priest to feel much smoother. This may be because the current stress test is capped at lv15 or because of the super high starting zone spawn rates (which blizz confirmed tapers off later on).

One of the big advantages of 3 mages is you can get away with arcane explosion spamming mobs. On a certain server I tested 1 war 3 mages and a priest in deadmines (believed to be harder than classic deadmines but I don't have #s) and arcane explosion clear speed was pretty good. Was able to hit 15-20k exp/hr in DM on the server that didn't account for the supposed easier mobs in classic or the bonus dungeon elite exp. AE spam is also relatively mindless so you can be a fairly effective warrior while AE spamming.