I started to tell you in-game but you said to post it so I shall.

You could add to the fighting/training - challenge/fun if you used speed as a direct factor in combat/training.  Ex: Someone with a speed of 1 wouldn't hit as fast as a person with a speed of 2. So the faster person would hit 2x (or if you group it in to categorys 1-10 spd attacks so fast while 11-20 attacks faster) faster than the one with a speed of 1.

Instead of speed just being a Req. for jutsus it would add to the diversity already available in training. Strength vs Speed (Power of hits vs Many small hits) and to further complicate this you could add more into the equation with stats such as evasion and accuracy. Furthering the available diversity in training and character types.

I might of had a snippet of a previous system of speed training and combat I had created if the PC I created it on didn't crash fry.

If you like the idea just Message me and I'll help you out in detail if you want.  I also have a ton of Naruto MIDI if you'd care for those as well.