To elaborate on my previous post, it's not just the upper limit, but also the expense. Wind Wall has requirements: (2)*Wits, (2)*Control, (3)*Resolve. Right now, Keiko has:
Wits 4
Control 5
Resolve 4
This means that her max Wind Wall level is 8, which would give her +2 Weapons/TacMove die if she chooses to level it. Note that XP costs are halved because I assume we level Zephyr's Reach to 10 beforehand. This is a 19XP sink which should also be factored in.
Zephyr's Reach 10: 19XP
Wind Wall 4 (+1 die): 5XP
Wind Wall 8 (+2 die): 13XP
To advance to +3 die Keiko would need to increase her Wits and Control to 6, a 34XP sink. She would then have to level Wind Wall normally, applying the XP discount partially due to advancing past her max Wind Release level.
Attributes: 34XP
Wind Wall 12 (+3 die): 32.5XP
So to sum up:
Zephyr's Reach 10: 19XP
Wind Wall 4: 24XP
Wind Wall 8: 37XP
Wind Wall 12: 103.5XP
You can see that there's a massive XP requirement to get from Wind Wall 8 to 12: this is (partially) the attribute requirement. We can butcher this requirement by spending it in conjunction with other skills: Wits (Awareness, TacMove, Stealth) and Control (Summoning), which should allow us to spend the XP with other things, and level Wind Wall itself at some future point. That is still a 32.5XP sink, however, and it honestly isn't worth it.
tl;dr Wind Wall can give the team +2 TacMove/Weapons die easily, but the third die is stupid expensive and we can't go above that. Asking Jiraiya for team-based mobility-enhancing jutsu is a reasonable favor. Though I'd want Keiko to notice that it shouldn't be the only favor, and that she should be focusing on other possible jutsu as well (Hazou got two jutsu ffs).
Side note: The damn technique doesn't move with you, and you have to cast it multiple times for a consistent boost (see: Inoue chasing Arikada in the Worm chapter).