Sarg,

If your impacts are above the (original) sights POA and apart (right barrel shooting right, left shooting left), In general, with a bore rifle, you need to slow things down a bit or go to lighter slugs. You are getting too much recoil assuming you are holding the rifle correctly. There is tons of convergence built into bore rifles, especially the 8 & ups, so they behave slightly differently from the nitros. You probabally know this, but thought I'd throw it out there.

How was accuracy as you worked up from lower velocities?

Again, I urge you to spend most of your time at 100 yards, that is the first sight and you will learn the most.