Accuracy is the same as the previous .45-70. Mine speaks with a tad more authority now.
If you're speaking of shooting a .45-70 in the .45-90 chamber, accuracy can fall off due to the .300" increase in freebore. If you seat the bullets out further, there's less jump and you may get better accuracy.
If you do this, be sure to clean the chamber really well; the crap can really collect in that .300 area.