Yep... the V-force is an awesome cage on the YZ 125's. It really makes a noticable difference. I have my reeds set to low tension - and there is a noticable gain in throttle response and the low-mid range power. I didn't really notice any type of effect on the top end power.
Shift-MX is right... if you set the reeds to the low-tension setting, you'll have to keep a much closer eye on them. Moto Tassinari warns you that if you choose this setting, the reed life will be significantly decreased (reason being is that the reeds open easier/more often). I have had maybe 30 hours on a set of reeds and found a chipped reed when I went to replace my reeds this past week. Many more intermediate-expert riders really like the high-tension setting and performance gains too, though.