Hi Finn,
I was hoping someone with better memory/understanding would jump in, but I'll take a swing at 'general principles'.
If memory serves, the guys who have applied the paper to hardware recently have said something like this:
The sharper edged lips are the most efficient shape where the flow is 'perfect' into the diffuser, and there's no spillage over the outside of the lip. But in our typical situation we need lots of flow at low airspeed (climb profile; high power) and less flow (relative to freestream) at high airspeed (cruise). If airspeed is high enough that some of the air in front of the inlet can't get in and must divert around the lip, the sharp edge of the lip will cause turbulence and drag on the outside of the airframe (at high airspeed, where it hurts the most). So, we can't truly optimize the lip. If a cowl flap will be used to increase cooling flow at low airspeed and reduce it at high airspeed, that means there will be significant spillage around the lip at high airspeed. So while the fatter lip is less efficient in the ideal flow situation, it ends up being better in the real world, because we must be able to cool at low speed and we want minimum drag at high speed.
There's also the 'internal vs external diffusion' issue. I think Bernie Kerr was the 1st person that talked to me about that. Internal (K&W duct, for instance) is theoretically more efficient, and could use a sharp edged lip, but it's really difficult for us 'measure with a micrometer; cut with an ax' builders to get perfect. So the safer thing is to go big on the inlet to ensure plenty of flow, and then throttle the outlet to match actual flow to conditions, with a cowl flap. Since we'll have a lot of spillage around the inlet with the flap closed, that means a fat lip on the inlet to minimize drag when there's lots of spillage.
Or I could have an advanced case of oldtimer's, and I'm remembering it wrong.
I hope someone will correct me if that's the case.
Charlie