What do you mean by device specific GSI? What's the point of doing this while GSI (Generic) means (for us) that you can actually flash it over multiple Treble devices out there?
If you recompile system image with device specific patches it will be basically device specific standard ROM (but with Treble which is basically not needed now) and not GSI anymore.
I don't get why are you people actually need "Treble" compatible ROM while existing standard ROMs are working great for now. Because they're Treble ROMs? What does it change? GSIs created using phhusson scripts are one big hack at the moment and their state is experimental. Of course they will work but they're lacking of device specific build.prop's.
Oreo one's are actually fully compatible with Lenovo P2 and stable because I tested them myself. Probably all of them (but all I tested) share the same device specific bugs. Even FP gestures works but this function needs to be activated by "setprop persist.sys.fp.navigation 1" after boot. The fix zip provided in P2 Mods subforum is actually copying script to system which will actually do that after every boot, nothing more. Google told on their AMA that they will make official documentation of making GSIs in the near future so I believe that will fix our current problems and device specific build.props could be generated and added to ROMs without touching system partition integrity state and recompiling them.
Until Android Pie ROMs will be stable as GSIs you will also not benefit too much from creating device specific system image because you will need to wait for Mike or someone else to create compatible vendor image and kernel (Probably Mike will be the first so you will need to wait for official release of LOS16).
Recompiling ROMs just for adding few fixes is a waste of resources. You need approx ~250GB of memory to compile and maitain one ROM, time (depending on memory you are using) and a lot of electricity.
In my personal opinion, creating Magisk module for P2 devices which will fix build.prop systemless-ly and add that command for activating FP gestures will be better alternative for now. I could personally look into it when I get some time.
If you still think that you need these device specific system images for every ROM at all costs, you could build them and maitain yourself or ask someone who will do this. Maybe Fullbustah will be eager to do some requests because he already built some popular ROMs on Mike's sources lately. You could also extract system.img's from his last builded ROM's and they will be device specific system images you are looking for
. The only problem is that I don't think he is willing to maitain all of them at once.
