If you have a problem with OpneERP then have a look at OpenBravo. You will find ad_org_id (a reference to ad_org, the equivalent of company_company table) in several tables, for instance in m_movement the equivalent of account_move.
Explaining (Postgresql) the move.period.fiscalyear.company SQL gives a score of 6,85 (25 steps !). Explaining move.company SQL (assuming there is such a column) gives a score of 2.85 (2 steps: table scan, sort). I doubt it will not have an impact the overall performance. The more I think about it, I guess the OpenERP limitation of 1 level in the dot notation is not that bad !
