Should I expect some troubles between my SpriteExt and base SFMP-API ?
Yes: you can't write it :lol:
If you do it 100% with OpenGL, you won't be able to use the sfml-graphics internal stuff (render windows, views, shaders, etc.) It won't be integrated at all like SFML drawables.
If you want to integrate it to SFML you have to inherit from sf::Drawable, but this class already implements what you want to get rid of.