A taller front sight will fix that for you.
Edit: I had a little math in here, but made an error remembering it for rifle iron sights. But here's the way I should have done it.
Inches High ÷ Inches to Target × Sight Radius = Needed Front Sight Height Increase
I'd run another group or two just to confirm you're really shooting that high (I know the anticipation of recoil can get me a little twitchy, and get me throwing shots high when I'm shooting SA for a tight group with heavy loads). You could just live with it as is, but 6" at 15yds is 10" at 25yds.
I'm not going to suggest this for competition purposes, but running a 240gr LSWC at 1,000 - 1,100fps might be a nice intermediate load that would bridge the gap between the two extremes.
This is all just food for thought. Do what you feel will best suit your needs.