Personally, have found that some people on high-index need vertical centration as well as horizontal centration. The refractive index shouldnt make much difference as you have been told. So another thing to try would be to lift your glasses up or try wearing them lower down to see if this makes a difference.(I'm assuming the pupillary distance is correct) If this makes a difference then you will need to have heights taken and the glasses made up again.
If your Rx changed, it will take some getting used to, especially if you have an astigmatism change. You should have them recheck your PD to make sure you are seeing out of the correct part of the lens. If your glasses are sitting too far off your face it can also cause some distortion. My optometry place seems to think that moving from 1.60 high index to 1.67 should make no difference, hope this helps.
Generally speaking, the higher the index of your lens, the more it will cause distortion. This is because higher index lenses are more dense than standard ones. The more dense it generally is. The lens's density is related to its Abbe number, or the amount of aberration it causes.High index glass has an especially low Abbe number, meaning this material causes the worst color aberration.Many people have no problem with the amount of color aberration they get from high index lenses. The people who notice it the most are those with very strong prescriptions and very high index lenses. Anybody with high index glass will notice at least some aberration.1.70 high index plastic has a good balance between thinness, light weight, scratch resistance, and aberration. This is the best choice for most people.
If you need high index lenses, it means that your prescription is a little bit strong. However, higher power lenses can cause distortion of
peripheral vision and may become thick and heavy if a large lens shape is used. However, if the lens becomes too small, the field of view can be drastically reduced. Bifocal, trifocal, and progressive lenses generally require a taller lens shape to leave room for the different segments while preserving an adequate field of view through each segment. Hope this helpful.