Hey Lise.
Yes, I have asked for lower interest for my visas but was only successful with my Vancity Visa and CIBC Select Visa. With them, I had to pay a yearly fee (can't remember how much) and was charged a lower rate of interest at around 12% or something. I had no problems with paying that yearly fee for a lower rate because the fee will pay itself out in a month or two. It was wayyyy cheaper for me. But now, I only have one Visa, the Vancity one with the lower rate.
I tried asking PC Mastercard but they wouldn't budge.
What you could do is transfer your balance to PC Mastercard. They have a very low interest for transfer of balance at around 4% or something.