The amount that a car seat affects rear visibility will vary from vehicle to vehicle. The amount a driver will notice the reduced visibility will also vary depending on each person, their size, and how accustomed they are to using their side mirrors.
If you find your rear-visibility impaired with a car seat installed, the first thing we would recommend is trying to install the seat in a different seating position, as some back seat positions will likely limit rear visibility less than others.
The truth is, however, that ANY car seat will impair rear visibility to some degree. And, as children grow and get taller, the impact on rear visibility will likely increase over time. We therefore generally encourage starting to adjust to using a car's side mirrors when a child is younger as it eventually becomes inevitable down the road.
Car and driver has a great article on adjusting side mirrors to provide full rear-view vision, which you can read here.