The statistics I've seen from about 5+ years ago suggested that the average improvement was about 2 or 3 lines on the eye chart. But with new surgical equipment available today, there is now probably greater improvement on average. I had ERM surgery in '06, and my vision improved by about 4 lines. BTW, it takes about a year to achieve your best vision.
But I'm not sure that it's very meaningful to talk about the "average" results of ERM surgery. If there are other conditions present before surgery, such as macular edema or a macular hole, this might adversely affect the outcome. Your surgeon is probably the best source of information about what you can anticipate post-surgery.
Best wishes for an excellent outcome.
The success rate will depend on how "success" is defined (return to normal vision or just keep from getting worse) and from surgeon to surgeon.
I would suggest you consult at least two vitreo-retinal Eye MD ophthalmologists. Ask them their success rate and what they call a success.
JCH MD