Kakashi and Itachi did not and do not have low chakra levels. Their techniques are just VERY taxing. Especially for kakashi, raikiri + sharingan and not an uchiha. Also Itachi's apparently low chakra and stamina was a direct result of his use of the MS and abilities like amaturasu and tsyoukomi (can't spell)
This is true kakashi and itachi dont have low chakra reserves. But compared to all the other op ninjas, they seem to, as those people have large chakra reserves.
To the Op
Sasuke is not getting a free chakra boost, from unlocking a dojutsu. It just doesnt work that way. He is appearing to have more chakra, but in actuallity, it is just his techniques are takeing less chakra to use. There are 2 ways this can happen.
1 Sasuke, has his chakra getting stronger(not getting more, but it just becoming stronger. For instance, the bijuu, have very dense chakra compared to a typical shinobi. Think of it like this. Fireball jutsu by shinobi A using say 50 chakra(ik measurements dont exist).
Shinobi B has bijuu chakra, and uses the exact same fireball, using the same amount of chakra(50). Shinobi Bs fireball, would be stronger than shinobi As fireball, because the chakra is stronger. THe fire ball would likely be much larger as well.
So essentially, this method makes for either stronger techniques with the same amount of chakra, or the same technique useing less chakra.
2 This method, is that simply his pool, and chakra strength have not changed alot, but upgradeing dojutsu, allows the sharingan techs to use less chakra. So simply sussanoo with ms, would take 100 chakra. Sussanoo with ems, would take
50 chakra for the same or better technique. But chidori with ms would take 50 chakra, and chidori with ems would take 50 chakra.
So in conclusion, sasukes chakra MOST LIKELY has not increased much. But his chakra has likely gotten stronger(mopre dense) AND ems reduces the amount of chakra needed for ms techs.
IT is HIGHLY unlikely that sasuke has had increased chakra from simply unlocking a higher dojutsu.