So I've taken a shining to playing support characters recently. Mostly just to give myself a break from feeling pressured to perform as well as everyone else on the team, kill-wise. I like using Janna a lot, Soraka being second.

Thing is, people keep expecting me to go solo the middle lane. Is that really advisable? I always decline and have someone else do it. I build my champion specifically to benefit the team and not to get kills, so being in the middle lane wouldn't make much sense. Not to mention, Janna has **** for offensive skills anyway. Her whirlwind is good, but it takes forever to set up, and Zephyr is awful and is more beneficial with its passive.

Tell these people that they are bad and they should feel bad.

AP soraka is pretty viable in any lane...but that's just because she's soraka

Karma mid lane and build AP carry. As long as your teamcomp has someone else to pick up the damage role late game, things go very well. At least for me that's how it usually goes.

Edit: It's good to build a durable AP carry, like with Rylai's and stuff.

Janna is a very versatile support, and you can certainly mid with her. AP Soraka is not really very viable anymore because she can't self-heal mana.

Somewhat related story:
I tried a few games with my friends where we went double mid, solo top, solo bot. Totally owned the mid. When I read your title, I thought you were going to talk about that XD
I advise that you try it. Support a mana-hungry ap mid.