Heres something you didn't take into account:

The time to kill mob + the time to recover mana/hp to kill the next mob.

Also, most places where people solo are minimal xp zones (IE outdoors).
With a good group you should (assuming you want xp) you should go to a more
difficult, and higher xp zone. From my personal experience I would estimate
there is up to a 20% xp bonus in some dungeons. (FYI the ones that seemd to
have the highest modifier to me are Unrest, Mistmoore, Dalinars and Kaesora.
Guk gets an honorable mention because I think the mobs there are very weak
for their level, thus yield good xp despite the zone bonus seeming not as
good)

At lvl 31, in a 5 hour night in Dalinars, I have gotten 6 bubs of xp! (Group
size was 4-6 people) Was it a freak occurence? Nope, got a little more
than 5 the next week (in the same 5 hour span).. Both nights, every person
in our group dinged (not all were there all the time), it was pretty
amazing. They tried out Nurga this week without me, I'll see next week how
good it is...