It can get that way sometimes, you'll finish up in a zone and be a little underleveled for the next area but over time it evens out by being able to do elite and group quests that soloers don't do. With the addition of a few new quest areas (eg Mudsproket in Dustwallow) and the xp changes post 2.3 I don't think it will have a noticeable impact over all. As an anecdotal comparison, my 5-box team all dinged 70 at almost the extact spot in Kopp's leveling guide as my last 2 solo 70's did.