for each extra character in your group mobs get:
Normal - +75% Health
Nightmare - +85% Health;+5% Damage
Hell - +95% Health;+10% Damage
Inferno - +110% Health; +15% Damage
As I recall, at lvl 55 wizard I had 4300 dps and 24000 health.
Now level 60, 29k health, ~11k dps.
Thanks for the information on the difficulty scaling!
Oh, and after your post on your stats at 55, I spent 100k at the AH today (after leveling to 56) and was able to get 5000 dps and 19000 health. Looks like the armor at level 60 is loads better than that of mid 50s too!