Well, I like that you took the time to do some math, but it's not black and white. As far as W goes, I, as Janna, use it almost off cooldown, so the loss in damage is slightly higher, but that's not even the big nerf. That's placebo for sure. Makes her slightly less oppressive.
The real damage is the MS, and not in the "how long she can get back to lane" department. I am an experienced Janna player, and I know her laning patterns and the way she interacts with other bottom laners. The MS she has and the MS she gets from her W are in such a position that enable her to do things like W-AA-backing off, that now with the -5 MS might not always be able to. The biggest impact of this -5 MS is in the laning phase, where every little detail in positioning matters so much. Even that 5 MS distance that you could be closer can make a difference in the laning phase, because it's all about the tiny details.
Honestly, I haven't played Janna since that nerf, so I cannot tell you for sure if it's a big hit or not, but before the patch, I made an assumption / claim that this will be much more impactful than what it seems, and it shows on her winrate. Nothing else was really touched to make the drop in winrate accused on it. For example, you could tell me that a counterpick support like Thresh got some buffs and people are now picking him more and that's why Janna's winrate dropped, but nothing like that happened, so the drop in her winrate is clearly because of that nerf.
Again, I haven't played her, but I do believe there will be some impact in the details of her laning pattern. Also, you need to understand that because of her W, the MS boosts she gets from it is also lowered, so the total MS she loses is probably more like 7 than 5, which is also lost from her passive damage, along with her W. You can see that slowly all these things can pile up and eventually you see how it can make a difference.