To give an idea of what the XP will be like, I rolled up a halfing rogue for max XP benefit on ragefire the other day and at lvl 1 a yellow con gave me 12% XP, I haven't tested phinny in a while but I want to say it was 2%.
This is hardly a quality XP test but it did give an idea. You won't be leveling super fast but it's decent.
I want to say phinny has a cap on % per kill which would make power leveling an alt on ragefire considerably faster.
I really need to do more thorough testing but it gives you at least something to judge it by.
On a level 1 Ogre SK, a single yellow was 10% (solo). Level 1 was all armadillos in Oggok, about 5 minutes. Level 2 was all white mobs, dropped to about 2.5% per kill. Level 3 most things were blue, seriously slowed down, (maybe 1% per kill?), just slaughter everything (Scarabs are always better exp for level) I looted everything, and had about 20 plat when I hit level 4. Burned it on Fletching. Archery speeds up kills immensely, most hits take 35%-50% of mobs health, I did 4th in half the time of 3rd. Keep all you Lizard Meats to hand in when you buy spells, and fire beetle eyes for grobb AC earrings. Both are free exp.
So far Ogre SK to 5 was fastest for me, followed by Troll SK, then Erudite Necro, Dwarf Pali, Wood Elf Ranger, and last Human Druid. (Troll would have been faster, but I leveled Blacksmithing to 92 so that burned time/plat). On the Troll I moved into UGuk at 6th, and could keep entrance, 1st alcove, entrance to crocs, and wanderer cleared with some effort, and zone hopping. Ranged from blue to yellow. (be easier with Ogre stun resist.) Some Banded made a big difference, AC is huge, don't ignore it.
Merchants in zone was a huge help, and a plus for Feerott, as were the copious netted drops for gold. Close to Innothule, so you have alot of potential hunting ground at low levels to absorb the crowds.
I am not a fast leveler by any means. I waste time by running to guards instead of dying pre 6th, I look for specific things to kill, I wander off to Grobb for no real reason, I craft, etc etc. A group in Feerottt/Innothule/Uguk could easily cut my leveling time to 5th by 1/3 or 1/2.
Just my 2 cents.