Hi Andreas;
=> with use of javascript and tirdd party controls (telerik). Inheritance adds more complexity in the translation process, and I believe it will make your application slower.
FWIW: Neither JavaScript, Inheritance or 3rd party controls have this type of problems in Appeon. Appeon even handles PFC based applications - and that's a lot of inheritance. Appeon produces HTML, XML, & JavaScript from your PB source code and has no issues like Webforms in handling this or giving amazing performance (quite often better than its Win32 counter-part).
The problem is really how Sybase implemented the Webform feature. Telerik's controls only helped Webforms work better but that ended up not being enough to help a poor initial technical design.
Regards ... Chris