just some thoughts:

he countered kurenai's genjutsu after seeing through it with his sharingan... so it doesn't necessarily mean that he can own her without sharingan.... if he couldn't see through the genjutsu then he might not have been able to counter it... he can own her without MANGEKYOU SHARINGAN.. cuz he did... but he used his sharingan as an advantage tos ee through her genjutsu... without sharingan eyes he would be... like kisame... a high S rank individual... which a jounin could put some fight against... though not a definate win for either side.... (Naruto has a lot of underdogs winning over seemingly stronger people)

which leads me to my other thought... if gai isn't affected by the Mangekyou Sharingan.. then he has some type of advantage... if itachi had no sharingan at all... it would be like my example with kurenai.... itachi would just be an S ranked individual like kisame... but without the mangekyou sharingan... he still has his normal see-through-pretty-much-anything sharingan and his S rank ninja skills.... just minus one super strong technique.... gai would still have to deal with all of that giving him less of a chance of winning.... however it doesn't give itachi a sure 100% win... and like i said in Naruto... it seems like the underdog sometimes wins over seemingly stronger opponents... so give gai some credit

as for my thoughts on people focusing on their best technique... well its pretty obvious that itachi has focused on his sharingan techniques... he had to get the stamina to use mangekyou multiple times(like sasuke practices with chidori)... he had to practice to learn the black fire thing(without practicing a jutsu... u can't develop one)... so he must have focused on his sharingan training... that said ... at his age... which is what.... at the most somewhere in his late 20's... it is highly doubtful that he is top ranked (the best) in every single category unless he spent all of his waking hours practicing ninjutsu taijutsu genjutsu and everything... so there IS a possibility that gai's taijutsu and speed could be on par if not better than itachi's...

I honestly didn't read every single comment because that's 11 pages of mostly flames... but I'm not taking any sides... I think that there are a lot of good points made here... itachi is still skilled at a lot of other things so he shouldn't be assumed helpless without sharingan... but without his mangekyou sharingan (which would have given him an automatic win against gai) he does not have a 100% chance of winning... he said himself to not underestimate gai... if he knew he could own him without even trying he wouldn't have said that...