I'm not sure I can agree with that, P-51's seemed to be able to shoot down Bf-109's of any marque at any altitude, I've never read an encounter report where the Mustang driver said that he "didn't engage because a 109K was attacking the bomber stream at 20,000 feet so... no joy, you guys are on your own".
Also the B-24's operating at roughly the same altitude, so referencing an earlier post, both the Lanc and the Lib had a 3.4% loss rate if I'm not mistaken, so... ?
I can see your point about adding some armor perhaps, but was the Lanc so flimsy it needed an extra half ton or so of it? Still not sold that the .303 would need to be replaced but perhaps so, I don't have a good enough frame of reference for that to make a judgement.
Buff makes a good point in the difference in tactics betwixt the USAAF and the RAF, which if we're talking a switch to escorted daylight bombing, the RAF may have to rethink it's position on the BOX v STREAM point.
The Me 109G14AS with an oversized super charger and water injection came into service about June 1944 which closed the performance gap with the P51 considerably. It wasn't until the Me 109K4 came into service in October 1944 that one could argue that the Me 109K4 could match the P51. It had the same speed and wasn't at a power to weight ratio disadvantage. The reality is the Me 109K4, at least part tolerably the Me 109G14AS was needed 9 months earlier, December 1943 when the first P51 started tentative missions thought there was no combat till January 1944.
The Problem for the German fighters was that the full pressure altitude (where the s/c can maintain 1 atmosphere pressure) of their single stage superchargers was around 19500 ft and above this speed and power dropped in a knee. The equivalent critical altitude of the P38 and P47 turbos was well over 25,000ft while the two stage Merlins were also comfortable at 25,000ft.
By forcing the Luftwaffe to intercept at 25,000ft the USAAF worsened the Me 109G6 and Fw 190 performance.
The reason Fw 190 pressed home their attack to much closer distances against US bombers wasn't just because the Fw 190s impressive fire power of 4x 20mm and 2x 13.2 but the heavy armour of the Fw 190 which could resist the US 50 caliber longer. The big radial engine with the armoured oil cooler protected the pilot along with 50 degree sloped bullet proof glass.
If it could resist the 50 calibre browning the 303 Browning with a bullet 1/4th the weight was going to have trouble despite higher ROF. The German MG131 13,2mm was reckoned 3 times more destructive than its 7.92mm rifle caliber analogue. The 50 versus 303 disparity would be greater.
Range was not such a big issue I think. I recall a German night fighter pilot recounting receiving accurate fire from a Lancaster rear gunner at 1100m. In those circumstances the smart night fighter pilots broke off to fight a less suspecting bomber. However the situation was different. Slow Closure rates exposed the German night fighters for much longer periods to return fire.
One would want at least 150 lbs of armour per gunner to protect them. Two less 500lb bombs.
Bomber stream tactics were no doubt somewhat effective during the day in saturating German defenses but they provided no interlocking protective fire. A "Heersusschoss" was an award to a Luftwaffe pilot for damaging a bomber so that it fell out of formation. It was considered and act of skill and bravery equal to a sucesful claim.
One of the reasons even US medium bombers had two pilots was the exhausting nature of formation flying.
So my view the Lancaster could be modified: 50 caliber guns on the rear, at least 1000lbs of additional armour. So long as they avoided the heaviest FLAK and had an escort and did not attempt to deep a penetration deep into Germany they could work.
Deep penetration raids against a functioning Luftwaffe would need two stage Merlins and likely formation flying and a copilot.