Prime-
Mine used to have a pretty big differential, but I have worked on it steadily. For me, I have found it works best if I switch off specializing between the two (do incline for 4-8 weeks, then flat for same and so on) I approach it exactly as I would flat bench, Usually the first week is adjusting back to form etc, but then I make gains on the lift I am working on specifically. Now I almost as comfortable with it as I am with flat. When I switch back the other lift it I have found that it usually has maintained its max or even gone up a little.
Both are up to well over 4 plates now and counting with only about a 10-15 pound difference.
I started using a shallower incline on my most recent block, but this system worked well for me at a traditional incline angle which I always used until just recently (though I put the bench in the power rack for obvious safety reasons and bad spotting angle of the regular incline bench)