2800 rpm @ 60 mph = 9.5% slip. That is not good in my opinion but I don't have experience with highway slip only at the track.

I have had converters slip as much as 13% at the finish line. My current Frank Lupo 9 inch in the race car slips 4% through the traps.

My new build will have a Lupo 9.5 inch converter (its currently in the street car). I will be running 4.56 gear with a 30 inch tire but do have a Gear Vendors OD for it. That drops the 4.56 to a 3.55 gear. It will be interesting to see how it behaves on the highway.