I have a spinal cord stimulator & have about 15-20% improvement in pain level from pre-implant. Is that percentage about average for others?

No. Most patients will receive at least 50% reduction in their pain. How much reduction in pain did you have during your trial? You may need some reprogramming and evaluation to make sure your leads have not shifted. If you had less than a 50% reduction in your pain during your trial then you should have never been implanted.
Spinal stimulator. With such a low % improvement I would ask why the implant was done? Did you get a lot of functional benefit during the trial? I look at degree of pain relief and more importantly functional improvement. Most insurance companies will not allow permanent with such a low % relief.
Variable. Discuss with your doctor, relief can vary from super to poor. Unfortunately your pain improvement is in my opinion below average. You should experience at least 59% relief.
SCS. If you only have 15-20% pain improvement from the trial, it is not a successful trial. At least 50% or more pain improvement is usually used for implantation criteria. We also look at your sleep hygiene, your function, your medication use. Please discuss with your doctor your concern.