You are not correct in believing that the t4i sensor is the same.
Canon has stated that they believe 18mp is the optimum photosite count for the best overall quality of a APS-C sensor, but that does not mean all 18mp sensors are the same.
The T4i sensor has changed to allow hybrid autofocus. There are some microphotos of it here:
Here is the 550D Sensor: https://chipworks.secure.force.com/catalog/ProductDetails?sku=CAN-1190&viewState=DetailView&cartID=&g=&parentCategory=a0u40000000QQstAAG&navigationStr=ListProduct&searchText=
The 7D Sensor: https://chipworks.secure.force.com/catalog/ProductDetails?sku=CAN-1160&viewState=DetailView&cartID=&g=&parentCategory=a0u40000000QQstAAG&navigationStr=ListProduct&searchText=
Here is a index to the image sensors analyzed by Chipworks. They do not necessarily check them all, someone has to buy the analysis. Normally, a analysis is purchased by the competition to help determine if a patent violation has occurred, for example.
Thanks! Nice research!