The example you mention, by the way, is not exactly a shining light for the Germans. Out of 9 Dorniers, 6 lost, 2 damaged.
Since we're at it, let's see this example in greater detail.
This attack took place on August 18, 1940, also known as "the Hardest Day" of the Battle. To put things in perspective, this was a typical day as to large-scale engagements, as I have pointed out in this thread and recently in another one: on the days where the Germans put up a amaximum effort and obtained the large air battles they needed if they were to sweep Fighter Command from the sky - they were soundly defeated, and with high losses. The kill ratio for the RAF on this day was 2:1, and we're not talking small numbers, the actual tally being 69:34.
The best thing that can be said about the unlucky 9/76's attack was that it barely managed to reach and attack its target without being intercepted (it actually was attacked by enemy fighters just as it was engaging Kenley, but unsuccessfully and the British fighters withdrew to avoid friendly fire). That's not because they were flying low: they had not been spotted by radar, but they were very accurately tracked by ground observation. It is, rather, that the British radar did spot the rest of the attack waves coming up at roughly the same hour. Their estimation was a 350+ raid. It was an exaggeration, but anyway the Germans had airborne about 110 bombers and 150 fighters at that time. No wonder that the 9 flying at 60 feet weren't engaged before.
Now on to the attack. The supposedly weak British AA defenses of Kenley immediately hit
at least four out of 9 bombers (according to dr. Price; other sources state that nearly all of the bomber were at least damaged straight away by ground fire). Even the PAC line, a last-ditch, desperation-borne improvisation, damaged the wing of one Dornier and brought down another (the latter, admittedly, had already been damaged by conventional AA, so it might be that if it had been untouched, the pilot could have managed to dodge the cable). Two of the Dorniers crashed right there and then thanks to the British AA defenses.
Sure, the attack was a success in terms of destroying most of Kenley's infrastructures, and much more importantly in destroying four Hurricanes that were on the ground, under repairs and therefore unable to take off and avoid the bombing. They also destroyed one Blenheim and damaged a couple more fighters in the air.
Kenley resumed limited operations in two hours, and was fully operational on the day after that. Naturally maintenance times would be not as fast now, with just one hangar standing. Work could also be done outdoors, which however would be more of a problem in another season.
To achieve this, the Germans expended 6 Dorniers, had two more heavily damaged (the last one wasn't untouched; it only suffered
minor damage), and lost 45% of the aircrew between KIA, WIA and POW.
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That's the actual history.
Now, let's suppose this attack is done by 9 Bf 110s.
The damage to the ground targets would be much less significant, due to the difference in payload. Usually, a smaller tonnage can do
more damage than a bigger one, as was the case with many ErProGr 210's attacks, if delivered more accurately. In this case, the Dorniers were extremely accurate, so the Bf 110s would achieve less against Kenley. Let's say they destroy just two fighters on the ground.
Now, the Bf 110s would be more survivable than the Dorniers - though not exceedingly so, and especially not in this situation. Let's say they are more difficult targets for the AA and entirely escape the PAC line. When engaged by enemy fighters, however, they are in a dire situation, between a rock (the fighters) and a hard place (the ground) just like the Dorniers.
So let's say the loss rate is halved; just three Bf 110s are lost, one to the AA; one more is heavily damaged.
Is it worth it?
Apparently, we have halved everything, so the ratio remains the same. But looking at the whole picture, it's a loss for the Germans. That's because in this ATL, at the end of June the Germans had operational 1180 level bombers, but just 219 Bf 110s.
6 out of 1180 is a 0.5% loss rate.
3 out of 219 is a 1.3% loss rate.
Also note that come Seelöwe, the number of level bombers isn't crucial. That's because the bottleneck is in the number of Bf 109s that can escort them; the Germans, for all the time in actual history and for all the time in this ATL, do not have enough Bf 109s for the number of bombers they have.