From: China Lake, CA
As OP stated he had 80 Zeros escorting 65 bombers and was opposed by 40 P-40s.
Had he set 30-40 of those Zeroes to Sweep he may have cleared a substantial portion of the CAP....of course with RADAR the ensuing Escorted Bombing Raid (assuming the strikes come in uncoordinated) may see even more defending CAP in the air.
So generally I don't like to launch bombers against a target defended by such heavy CAP. Instead I sweep for several days to try to clear the skies of defending CAP...then I send in the escorted bombing raid hopefully catching the fighters damaged by the sweeps on the ground.
Works well early on ...at least until your Oscars meet the Hurricane.
Just want to clarify something that Tree has already dutifully mentioned. To be specific setting a portion of a would be ESCORT to SWEEP does not guarantee that the SWEEP will be conducted BEFORE the raid those A/C would otherwise be escorting. This is obviously the intent, when sweeping, but coordination is a key factor in whether SWEEPs are flown before, during, or after the strike....or even at all! You STILL need to consider forecasted weather, leader ratings of all units involved, basing (co-location helps, and distance between coordinating units does not!).
If you are setting units to sweep from a large AF with all participating units there, an Air HQ with a Decent Air commander, good weather and short distances to the target in question, these factors will be the best case for a well coordinated, sweep of an enemy Hex.
If you are having trouble, try to achieve as many of the things as you can above, OR consider prepping the battlefield, by sweeping with strong Fighter units BEFORE you attempt a sustained Air campaign against well defended targets.
IN PERPETUUM SINGULARIS SEDES