Solved! It was a metadata little issue because it didn't make any sense. I've re-linked the clip to AMA and works perfectly with the right metadatas.
At least this experienced showed me that if you disable AMA, import P2 will reappear in the menu wich I ignored that fact.
Then it also showed the excelence of the convertion to DNxHD (in this case 175x), really really good quality footage, no slow-down.
And then I've been very impressed by the fact that if you link via AMA a (this time) 720 P2 material and boost it into a 1080 project with DNxHD, it bloody stands still, almost unnoticiable, no softening nor visual quality loss. (unless you are watching the movie with a loupe in front of the screen)
It gives me more confidence to ingest 720 footage on a 1080 timeline in Avid.