More studies reveil the same.
With the med open to the Axis powers, which would be the strongest military power in the world in 1941 without the UK fighting, there would be no stopping them entering the Black Sea.
Even if the Turks would have tried such feat, I remain doubtful that they'd been able to, nor even wanted to.
Now, Gallipoli was about the exact same deal, and the Turks held their land at great cost, - in order to deprive the Russians of supplies from the western powers.
But that was before concepts like air power and airborne power came into the equation.
So, in short, with UK out of war with Germany, lifting the Naval blockade, the maps and entrance to the USSR would have looked entirely different.
One northern route from land ending in Moscow, one southern route from the sea and perhaps through Romania and Besarabia, beginning in Ucraine.
With no Balcanese or N-African campaign, the Axis could have launched their complete strength some month earlier, notably in the south.
With considerably more strength, twice the airforce, much better and swifter transport, extra time, Naval support, and no detour between the Northern and southern fronts of the USSR, I have little doubt that the Axis would have had the USSR in deep doo-doo well before the winter settled in.
Now where's Viking and Boroda when you need them