A couple of points: I don't think "80%" of the German war machine went to the East. Note that the US and Britain bagged about the same number of PoWs in North Africa (Tunisa) as the Soviets did at Stalingrad. I would estimate that the ground force distribution was closer to 60 - 40 East - West. The bulk of the Luftwaffe remained in the West as well with the East Front being secondary for them for much of the war.
I also think that Stalin and Stavka recognized that the US and Britain did open a second front in their invasion of Vichy North Africa and then into Italy but, Stalin still agitiated for a larger ground committment on the continent and for good reason. It would take some pressure off the Soviets.
And, I would agree that Kursk was the truly decisive battle in the East. Germany's attack failed and the Soviet counter offensive was the first truly massive and successful one. Its scale makes the previous offensive against AGS and 6th Army at Stalingrad look pitifully small by comparison. It is also an indication of just how massively the Red Army had grown since its initial defeats in 1941.