Itachi should easily outscale and overpower.
Tobirama, alive, should be below base Raikage in stats while Itachi is roughly around Kakashi who operates at the level of V1 RK. With speed being the one thing Tobirama excelled at in stats, that leaves him with nothing else to contest with in terms of stats.
In the powers department, Itachi specialises in very quick death strikes and tactical prowess, pressing home his speed advantage while covering any vulnerability to tactics. On the other hand, the best powers of the Hokage require some preparation in reanimations and teleportation marks. Even supposing he starts prepared, which would allow him to waste some moves of Itachi with reanimated meat shields and escape options should he be hit, he would still be vulnerable to illusions and simply force Itachi to do multiple castings of the Black Flames to deal with marked kunai and the reanimations. That, putting aside the full level difference in their speed. And the way the Black Flames spread would prevent Tobirama from even marking random points on th ground.
It would be entertaining, but only if the Hokage is granted some benefits he does not normally have. Itachi would have to have vastly inferior powers for the HK to make up for the speed difference, and he is actually easily superior in that regard as well.
