I wouldn't be able to speak to whether it would impact most negatively on a particular income group. It's certain that when you go to a flat rate, the vast majority of the reduction, if you will, the proportional reduction, is in areas of low unemployment than in areas of high unemployment.
The only income tested aspect of EI is the family supplement, which increases benefit rates to those with children and who have a demonstrated low income. It can increase the replacement rate from 55% to 80%.