.but L is no doubt a Perceiver....
I have actually begun to doubt that. L is far to forward with his reasoning IMO. Though he has a quirky side to him which displays a few oddities, I see no signs of procrastination or disorganization. When thinking outloud, he always has a solid and set-in-stone foundation of reasoning which he easily explains. INTP's will come to a conclusion, but have trouble explaining how.
Also, L was at the top of his class in school (unless this part was simply forged to allow him into Light's college). I have yet to meet an INTP that has a willpower/attention span to have such a high performance in a seemingly meaningless task. A J is far more likely to pull something along those lines. Upon L's emulation of human thought, though it could be considered 'perceiving', it must be considered that he was part of a special school which trained children to become as such. That could have been a learned trait, not exactly natural.
As for Light being INTJ, I suppose it is possible. A good friend of mine who is an INTJ, we are constantly debating about something. He enjoys to take whatever I say and grammatically correct it or scour my words for a hint of inaccuracy (though the intended message that came across was completely accurate). We get along great...we may just make others around us confused. Rather funny, actually.
Anyhow, I would definitely type L as an INTJ whie Light is a bit more vague. His idealism is rather...interesting and very overpowering. For him to have such a need and sense of responsibility to become a 'god' of the world, that is not exactly a logical basis. I do not think that fits into Te or Ni. I would argue it could be a very skewed Fi.