So I e-mailed NGK regarding the difference in the plug and power gains and this is what they said...
"Hello,
The difference between these plugs is in the firing end design. CR9E is a standard design single ground electrode plug, while CR9EK is a dual ground electrode plug. CR9EK has a deeper projection into the combustion chamber and may cause a clearance problem between the plug and the piston. I do not recommend CR9EK. For a safe upgrade for this motorcycle, I recommend CR9EIX stock no. 3521. This plug should provide easier starts, better performance and efficiency and longer plug life.
Kristopher Zdral
Technical Support Representative
NGK Spark Plugs (U.S.A.), INC."
Now its not just a ? of spark but according to mototuneusa...
"One reason the CR9EK's or CR10EK's make more power is because they take up more space in the combustion chamber, which increases the compression ratio. The second reason is that the spark is .040" / 1mm deeper into the combustion chamber. I think that increases the burning efficiency at high speeds."
So if there is no clearance problem with the CR9EK it makes sense that you are ganna get some kind of horsepower increase, provided that you change your plugs on a regular basis.