That could work quite well. Given that some spelldances already alter the extending rules it wouldn't be that big a change for a given dance to change something that minor. That said I'd need to put specific rules for the choreographed spelldances, likely in the area of that you count all extensions as being at the DC increase of the current portion.
While that's true, I'm not hugely concerned about the tornado in that situation, as a low level character will then find it difficult to keep up with subsequent DCs to sustain it for long, while a high level one won't be that big a deal if they throw out a tornado. That said, as I noted before the real issue is the out of combat utility use, such as using an AMF to get past magical traps or the like.
Ah, I see your issue there. There is actually a reason for that. Specifically, the increase in routine already has the same effect as the extend effect. Really, the extend effect is to increase wind strength past those of the routines. Also do note that increasing the routine is optional, so if one wants to keep it at the current routine but still have it impact larger creatures they may do so (for example if they want to keep it more a gust of wind effect rather than making it a tornado).
There is also the fact that extending it has the benefit of keeping the spelldance going, and allows the dancer to change the direction the wind is blowing (particularly useful if the dancer moves and wants to keep it focused on her enemies).
Still, it just occurred to me that the manner I altered the wording is still a bit off from the intended effect, and I may alter it slightly. I may also change it to adjust the size creatures are treated as every two extensions rather than every three.
Owrtho