Without venturing too wildly into speculation, it is a tangible and known pitfall that due to urgency in the Mediterranean virtually the entire air support over the Moscow district was withdrawn late41 with the reassignment of Luftflotte 2. As it turned out the Soviets had as many serviceable aircraft flying from good fields around Moscow that the Luftwaffe did spanning the entire Eastern Front in December-January, it was a terrific disadvantage.
What happened outside Moscow was the German drive effectively turned into a trench war (due as much to the attrition and condition of the mechanised force as it was Luftwaffe absence). If it had Luftflotte 2 and they managed an acceptable serviceability rate it is possible the likelihood of successfully taking Moscow would've dramatically increased even under trenchlike conditions, perhaps as a last ditch all out attempt, the one Hitler demanded in the sheer absence of air cover or any decent artillery.
But the argument against is that serviceability for the Luftwaffe on the Eastern Front that winter was as low as 15% due to being poorly adapted to the harsh wintery conditions. Soviet aircraft by comparison could top 60% in the same conditions.
If Luftflotte 2 had've been there 85% of its number would've been useless anyway, so they were better used elsewhere. I believe that was the reasoning. Even lend-lease Hurricanes were rebuilt using Soviet made consumables (like oil) that were better suited to the climate, or else they were stripped for parts to use in Soviet made aircraft (like armoured seats).
That was really the greatest danger period, where the absence of the Luftwaffe was most noted on the Eastern Front. Other than that you're really talking about JG27, JG53, Fliegerkorps X and a smattering of attachments and elements being really absent elsewhere due to their commitments in the Mediterranean. It's not really a conspicuous drain on resources, the Afrika Korps is really just a brigade attachment given independence and regional command authority, it's bigger on paper than it was IRL (smaller than divisional status).
The massive drain that Africa turned out to be was in supply and logistics, it was a massive, ever increasing commitment on that level, and whatever of the German transport force that wasn't destroyed at Stalingrad, was in the Mediterranean the moment the British cracked the communications codes with Ultra. Then it was just a slaughter, vast shipments sent out and sent directly to the bottom of the ocean, or crashing from the sky in flames irrespective of escorts.
Africa and the Mediterranean were like a seeping wound, not too serious in terms of initial impact but it just kept bleeding and bleeding, opening ever wider until threatening to kill the host. It mightn't have even been so bad if Hitler just gave it up when pushed back to Tunisia, but whilst previously treating the Mediterranean like a distraction he suddenly had a change of heart when all was lost and made a desperate full commitment of resources. That was very costly on several levels.
Some say his piecemeal use of new Tiger tanks in Tunisia, not to mention the latest and best German heavy artillery handed the Allies premature preparation of what to expect in western Europe, and that the Tiger could've had a much greater impact if held back until available in good numbers for a large scale commitment towards specific strategic battlefield objectives. It's a nice example of what Africa wound up doing for Germany, it was entirely a liability in the long run.
Other than that issue of air cover at Moscow in 41 though, I'm not really sure that you could say any significant bolster would've been available on the Eastern Front, German forces in the Mediterranean weren't huge in terms of numberical presence, and already everything that had been committed to the Balkans had been shifted to take part in Barbarossa, except JG27. Logistics for the Eastern Front would've been better, particularly in the crucial period coinciding with Stalingrad, perhaps Rostov would've had supplies von Kleist and Ruoff badly needed but it was strategic errors causing the turnaround there, use of numbers that weren't really available and wouldn't have been with or without Africa.