I'm going to try to do the math on how much Janna can do with 1 W during laning phase (see below for assumptions). I don't play her, so I'm sure I'm going to make some mistakes on interpreting how some of her stuff interacts and what sort of runes she should take. For the purposes of this, I am going to aim for a Janna that is building to do the most damage with W early.
Assuming level 6 -> tier 1 of passive, 3 points into W, boots, and a frostfang, plus sorcery runes (Aery, 30 AP from bonuses plus scorch and other bonus damage runes). Runes are a bit vague cause I don't want to open the client to see what sort of optimizations I can work out. This is a quick and dirty version.
Rank 3 W does 170 + .5 AP + bonus move speed passive. Her passive grants her 8%, and at rank 3, W also grants 8% plus 2% per 100 AP. I admittedly don't know if that portion of move speed is lost immediately on cast (and thus not applying to W's damage) or not, but I'm going to try to go for the worst case (most damage) and you Janna mains can let me know how wrong I am.
Janna at that point prolly has somewhere around 50 AP (30 from runes, 20 from frostfang) - boosting her W passive move speed to 9%. She has 320 base + 25 boots + 17% bonus (assuming passive and w stack fully) granting a total of 379 move speed or 59 bonus move speed. She converts 15% of that into damage (8.85)
So W becomes 170 + 25 + 9 for a bit over 200 pre-mitigated damage. Adding on aery, scorch, and other damage boosting effects from runes (move speed increases, more damage at full health, take your pick) could add perhaps up to 100 additional damage conditionally.
I would assume this ability hit you twice (it has about a 7 second cooldown at this point assuming no cooldown runes), since it doesn't seem like this ability can do 400+ damage at lane phase levels without something being terribly wrong.