Since the invasions started I have level 3 characters from 100-110. Each of them I used a slightly different method to test out what is best.
DK - This one was purely leveled from invasion events. This one was the fastest by far. It's true that I was sorely under geared at 110, but after getting some AK and doing the broken shore, by the end of the first night I was pushing ilvl 830, he's up to about 855 now, no legs yet :(
DH - For this character I would stop after 2-3 invasions and do a bit of each zone and maybe an order hall quest. I found this method worked better for initial ilvl. It was slower in hours per level, but faster in days as I could obviously get more XP per day. This character was easier to get to 850, but a leg helped that move along faster.
Mage - This character I leveled up almost purely by invasions, but then the last level was done via quests. This helped my ilvl out, but not enough for it to be a real game changer. This one is not up to 850 yet.
Pros: -Fast leveling -Order Hall Resources -Limited access to prevent binging(Some days I need this)
Cons: -Little class hall progression -Low gear drops -Little character investment(for me)
Overall: I think the biggest factor that affected leveling this way wasn't while leveling ilvl or at the end, but the class I was playing. I did struggle a little with DH and DK with low ilvl, but they are survivable classes and it didn't impede me. With the mage, I constantly felt like I was outclasses by even non-elite mobs. This slowed me down a lot as with my mage I would die a few times per invasion quest, with the other two it happened maybe once an invasion.
Getting gear AFTER hitting 110 is really very easy with just doing the broken shore event, and then just doing another invasion or two for the shards.