You could try relaxing and/or conforming it to a smooth surface after subdividing if that's practical in your case (don't think so), but generally such harsh topology changes on curved surfaces will show artifacts and subdivision doesn't help, as you found out. (I think patches and Nurbs might be able to do variable resolutions, but they aren't practical.)
Haven't checked the file, but try to find a different solution to increase resolution, perhaps with a better loop flow, or think about how you can have an overall more even resolution.
