So in a situation like the Breakout and Pursuit scenario where the Axis have some serious unit stacks, is it best to 'prep' for a turn before launching a big attack on one of those stacks - e.g. do some interdiction in and around the hex, as well as some unit attacking?
Since you have the airpower, I'd opt for a 2 mission solution. Use FBs and plaster those hexes and the surrounds (ie a 1 hex wide mission) with interdiction. That will hurt any attempt to gain supply and cripple them if they move.
If you want the hex (ie its on the front line and you need to go via that hex), then mix this with a 'unit' mission. What this will do is inflict a lot of disruption/damaged elements. The advantage is these elements then cannot take part in any ground combat, so the stack is actually weaker than it appears - you'll see this in the combat results as a massive drop off in effective combat values.
For 'unit' missions, I often find 2 engined bombers are quite good.
If you lacked the massive airpower of the Western Allies, and could spare the time, I'd simply use interdiction to weaken them before attacking in a couple of turns.
For unit bombing of highly stacked hexes, you may also want to use heavy bombers (dropping high explosives). The Allies used 8th AF and Bomber Command to bomb the defensive line prior to an an attack on several occasions besides Cobra (in July 1944). In most instances, more than 500 heavies would drop their bombs from less than 10,000 ft.
I also use the heavies as prep bombardment of invasion beaches. When carpet bombing units with heavies, I usually bomb on day 1 and sometimes day 2 of the turn of the ground attack. Heavy bombers are similarly useful for hitting surrounded units in a fortified port, although I often bomb the units for 2+ turns before sending in the ground attack (supported by 1 or more naval gunfire support TFs).