If I was your DM...
...which I am not (turning to address bleedthefreak)...
...Gromm is...
I would say that the cap is at +2 for adamatine weapons, otherwise it is too useful as a weapon in antimagic (or dispel) for the stated price. I think the Wizards took half of the maximum enhancent allowed in the game (half of +5 which is rounded down to +2) to express the inherent "magical" nature of adamantine metal.
...which I am not (turning to address bleedthefreak)...
...Gromm is...
I would say that the cap is at +2 for adamatine weapons, otherwise it is too useful as a weapon in antimagic (or dispel) for the stated price. I think the Wizards took half of the maximum enhancent allowed in the game (half of +5 which is rounded down to +2) to express the inherent "magical" nature of adamantine metal.