I just installed FFmpeg/FFDshow codec to try if they could work H264 with Opera
Opera can't use them as far as I know.
Do you mean I will unintall them ?
No, I mean that Opera can only play H264 (and some other codecs) if Windows can play them natively. In other words, Windows should be able to play those codecs just after having been installed.
Problems Detected
Did you check the whole page? Real problems are usually listed on the bottom of the page?
Also, are the items under 'Graphics Feature Status' with 'hardware accelerated' status? At least most of them.