Itachi would never be thrown off using his mangekyou...Kakashi gets thrown off because he isn't a uchiha, Itachi has uchiha blood thus his techs don't have those kinds of side effects... And i also can't agree that gaara has more experience.....We never really got to see what he does......how many fights he got into...But itachi has been in anbu since 10, and got promoted to captain at 13.....
I Did however take a look at their databooks, to see how much missions they did
Gaara : 34 official missions in total: 0 D-rank, 9 C-rank, 8 B-rank, 14 A-rank, 3 S-rank.
Itachi:340 official missions in total (jesus christ :S): 53 D-rank, 152 C-rank, 134 B-rank, 0 A-rank and 1 S-rank.
Itachi being 13 using such a powerful jutsu is guaranteed to have ill effects on him, even 16 or 17 year old Sasuke had pain in his eye and head after using Amaterasu for the first time. It's sure to have major effects on a 13 year old Itachi with chakra/stamina issues. That's why I mentioned Kakashi because
they both have stamina/chakra issues. If Itachi had major Stamina issues as an adult just imagine it as a 13 year old child with no testosterone.
Itachi has more experience with missions because he was an actual shinobi in Konoha while Gaara was used as a secret weapon more and he became
Kazekage at a very young age. Gaara has world class experience such as fighting; Deidara, Sasuke, Madara, Muu, 3rd Raikage, Mizukage(3rd), Naruto, Kimimaru, and his Father.
See for yourself? him in anbu since 10 was even more experience than gaaras
16 years of living