I didn't think there was really much further you could go. PDI is pretty much top of the tree irrespective of all this marketing bumf the brass plated barrel manufacturers produce.
Edgi steel maybe?
What BB's are you using? Maybe upgrade those if not already using "good" ones.
Edgi stainless is where i was thinking, she mostly gets run on geoffs .32's but i occasionally give get the .48 super precisions when i really want max groupings.
Like i say, i want more but its hard to think of the next phase at this point.
Are we talking sniper rifle here? What rough FPS are you at if it is? There was a very good write up on a-nother forum about BB quality and consistency, whilst Geoffs Supers were good they weren't "the best". I *think* BLS 0.4's were about the best under the test conditions for size/weight/consistency/shape etc but I'll try and find a link for you.
EdGi stainless though isn't going to be a noticeable amount better than your PDI.