Replaced mine a couple of years ago after the original one split one day as the top was folding down...
The new one shows slight deformation when being raised after being down for an extended period, but eventually assumes its normal shape...especially when exposed to sunlight.
...I would think any deformation problems would be the result of the installation.
When the top is up, the window plastic should be taut (side windows excluded). And except for slight scratches, your rear vision should not be impeded by any deformation...