As far as I know, it's a "still under debate" Thing, i.e. the yay and nay sayers of the porphyria theory rage on, but I'm anything but an expert. What's not under debate is that he did attempt to strangle Prinny.

Incidentally, the current Hannovers, true to form, are carrying out a bitter Father-son feud which has now gone to court. They're both called Ernst August (the older one was married to Caroline of Monaco for a while and infamous for beating up reporters and pissing in public when the big fair at Hannover was opened), and if you want to employ Google translate, you can read one typical article about it here.