First of all Edu, don't you think that if everyone starts to say what different class should get a HP Boost and which doesn't, this topic should get out of purpose quite easy. For example if now I say that X class needs a boost because of Y and Z reasons, I won't give my opinion about your suggestion for clowns.
So what we will have here is a list of all the classes that the community thinks that should get a boost, and we won't have any argument from the community from one clear and straight topic, just different ideas about different classes (topics).
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Now on the topic, It has been a while since I don't play Clown, but I always thought that clown could use some more HP since all that you said is true, they are vulnerable to a 1 shot asura, and if they go on reducts items the HP goes even lower. I know that some people might disagree claiming that a well played clown is some kind of war machine, I agree with that the clowns are good, but leave them behind just because of that is just not fair.
So +1 the suggestion.