What Roger says is indeed true. It is also easier to get called strikes on harder curves with less movement. However, a significant percentage of the college coaches I have spoken with this past year like to see 12-15 mph reduction for a curveball. Kershaw and Wainwright, two pretty good curveballers, often have 18-20 mph reduction off the fastball. Your current differential is less than 10, which appears to be almost slider like. I am not saying you should necessarily lose your current curve but a slower 12-6 will be a useful addition if you can get a feel for it.
I you look at the effective velocity matrix on Perry Husband's website, you will see that moving diagonally through the hitting zone maintains the same velocity profile. Moving vertically will change the effective velocity within the pitch.