Hi!
I already use WHFRP rules for "conan" games (more than 15 years), and i think it's very suitable for Hyboria.
My players love the simply, quick and bloody rules, they also like the critical hits. (Opponent's head flies off in random direction 2d6 feet away and some other nice stuffs) 8)
Of course, there are a little work with the system, first you need to throw out about the half of the careers (rat catcher, mentioned before for example), and you need to add some "never seen in the Old World" basic fantasy careers.
The characteristic and the skill (+talent?) system almost perfect, just few things needed to clean or change (in name). Same thing for weapons and armors, various equipments.
I never played the original warhammer, and i don't know how "darker and grittier" it is, i just saw some pictures in the books, but maybe Oly is right, especially if someone knows (and like) the setting of WH.
Magic problem: first, i never allowed any of my players to play with a magic user character in hyboria. When i use spell user enemies against characters, i only aware the statistics of the given spell, i always narrating the visible (or feelable) part different. Players will never know what they got. And WHFRP 2nd magic system and spells is much better for hyboria than the 1st one, there are only a few names to change, that's all).
As a gm, my favorit thing in WH is the career system. Think about Conan's careers...
At last, sry for bad english.