HPL2 vs HPL1 vs other indie engines

[Hannibal]

As Urkle said already HPL2 isn't rewrite from scratch. most parts are just the same as HPL1

No, it uses a new engine written from scratch

My mistake. It was a user, not a developer, who had said that. I found an interview from Jens in 2009, confirming there was some code re-use, and talking about HPL2's new capabilities:

Engine wise it is quite different and there many parts that have been rebuilt from the ground up. The Penumbra games uses a portal system to do occlusion culling (check what objects not to draw) that placed a heavy burden on the artists. In Amnesia we used a fully dynamic culling system based on a technique called Coherent Hierarchical Culling, which does not give any extra work for the artists and give them more time to make nice graphics instead of worrying about performance issues. Another example is the shadows that were stencil (shadow volumes) based in Penumbra but is done with shadow maps in Amnesia. There are all kinds of new effects added as well, like Screen Space Ambient Occlusion, proper decals, etc.

so far there is no bench mark utility for HPL1/2 available and making comparison against other engines are not possible as many things aren't equivalent and etc...

That may be true, but I'm certain someone who has worked with other game engines and used HPL or HPL2 would have some insights into the differences between them beyond "It's written specifically for this sort of game". Well, now that I've read the interview, I know the original HPL started out as an engine for a very different sort of game!

Still interested in whatever specifics others know or can make an educated guess about.