my early assessment: As an support, she excels across the board, a decent heal, great amount of cc, great initiation and disengage
My main concern: her Q mana cost for the damage output is too low....a prime example of comparison is Janna Tornado. It serves as a disengage just like Nami...Nami gets a longer stun for less mana and high base damage compared to Janna. I understand Nami Q is a skillshot thats harder to predict and act than Janna's tornado - but if you take into account equal skill, the damage output for the mana cost for Nami is superior. Reason as to why I state this issue is with her max base mana, she can dish out 5 Qs. Compare this to Janna's Q (Tornado), Janna can only dish out 3Q. Both champions have same base mana regen.
My own personal suggestion to the problem with 4 options (preferences in given order):
1) Increase mana cost of Q to probably 70 or 75.
2) Decrease base mana regen by slightly. (Can't get a hold of a number that would be considered balanced)
3) Decrease the stun of the Q to 1.0 second (Seems adequate number)
4) Decrease base damage on the Q. (Can't get a hodl of a number that would be considered balanced)
**All stat comparison im using for Nami is speculative that was released on Surrenderat20.com. Janna stats used on LolWiki**
Overall: great support, kit flows relatively well, possibility to be abused as a mage depending on Q is approached. [Similar case to Lulu].
**Dunno why im being downvoted for the Q. If I can play 5 games where i land 3 or 4 out of my 5 Qs on an enemy adc, ontop of that, I'm able to do more damage than Janna Tornado uncharged, that is a real concern. You might play her as Support but not everyone will. Hence why I feel testing her for things beyond her support capability; testing Nami as a mage caster in mid lane is a good experiment between determining how can a player can abuse the kit [something as PBE tester should evaluate ontop of her support role]. A prime example of when PBE failed to look at Lulu resulted in players in the regular client to abuse her Glitterlance. I myself dont want to go through that again, nor I hope you dont either.
*** I retried her as a defensively AP mage (140AP: doran ring, tier 3 boots, liandry's lament, frozen fist) alone with just assistance from new masteries page [no runes nor blue buff, baron buff and red buff] I could solo dragon. Q-E-W (to heal), repeat until dragon died. [I killed it but with around 100-200 health remaining. I know this isnt optimal but I dunno any one support can take a dragon on his or her own even building AP]. This is another serious problem that Nami AP ratio or base damage on the Q should be of a concern. [I realize you acknowledge that ap ratio might be tweaked.]