True, I shouldn't consider the charisma. But I do because of this, and I only know this because I attempted to make the character this way. Let me start.
First I reduced IQ to 8, assuming he would be a little numb skulled. Then I realized that all my most important skills were gonna be based on IQ. So I finagled it, and found the talented advantage and thought it would fix it all. Taking four levels into it, I found it cost me 60 points. I wanted to get to where I had a base of 12 in those skills, then it hit me that I would have payed less even when calculating the 40 points I received for lowering my IQ if I had just raised it by two. 60 points compared to 40, even after the points back I've spent twenty. Then there is still the matter of my will that would need to be raised due to the fact that I don't see my character any more weak willed then normal just because he's not smart, and also my perception because I have decided that just because he's not smart doesn't mean he's not attentive; and the smooth operator doesn't help me with detect lies or body language!
Total it costs me 50 points more then it does the person that just bumped his IQ twice... I'd say that dampers the concept. I would have been happy if we'd have just broke even.
Also, competent and multi talented are two different things. I have never had trouble with and enjoy new world of darkness, so when I hear someone has trouble with the system I offer advice. Apologies if that offends you...
EDIT: I also feel the need to add that I'm not trying to argue with anyone. I had a terrible day at work, the last thing I wish to do is argue. I was only attempting to help a fellow gamer out with their troubles, just as Thurfir tried to do with me and my GURPS troubles. Apparently neither attempt had any success tonight.
FWIW, I am trying to help you solve your problems within GURPS without having to learn a new system. It started as arguing that the disincentive is way less than in most other systems, but it seems the discussion is long past it.
Now, excuse me, I think your calculation is still off.
You have 150 pts, lowering your IQ to 8 gives you 40 points, so you now have 190. Buy that back to a base of 12, and you still have 130. For comparison, the guy who bumped IQ twice would have 110 left at that point. You can buy what, 4 levels of Charisma more and totally outclass him in your speciality? Then you follow the advice from the book and work with the GM to get a variant talent package, say Born Liar, that also covers Body language and Detect Lies. I'm pretty sure they say the packages are an example, not the only ones possible, and you should customize them.
Then you use the 20 points you spent to get your Perception and Will back, and end up with quite less of a difference. Sure, your character is less competent about mental tasks than the guy who is just smarter, but again, that's what lower intelligence is supposed to be. It is going to be more work if you want to compete. Again, I find that normal, and would only take that concept if I wanted to roleplay someone who is disadvantaged in related activities, but still on equal footing when it comes to cheating people.
EDIT: Basic powers of the witchblade are as follows: Sword like blade that extends from the wrist that is able to cut through steel. Increased strength and agility, ability to leap huge distances, body armor that protects the wielder from otherwise killing blows, heightened reaction time, reverse nervous system to where pain is pleasurable, and the ability to bring a blade from any part of your body.
It's gets many more as the player increases, but damn! It's hard to set those down in a way that works out well on GURPS.
Well, I'm without my GURPS books in the foreseeable future. But I remember having made weapons that did feed on the user's vital energy, so there's some way around this. I just can't remember it off the top of my head.
However, I do have one suggestion before you write off GURPS to invest in another system.
Why don't you go to the SJG forums
and ask them how to create the Witchblade? If these guys can't help you, nobody can
! But since the developers post there, I'd say you stand a pretty good chance!
They also tend to take "GURPS can't model this" as a challenge...
Personally, I've always received pretty good help on such matters. Note, if you don't have GURPS: Supers or GURPS: Martial arts, mention it in the post, as most examples might be from there
. So, just go there, register and start a thread!