In GT6, you can only clear all your laps records in time attack via a button in the main menu of the "before race menu". So if you run a very hard to break record with a "slow" car, you delete it when you enter the same track with a more powerfull car. You should be able to make justice to low pp car records. In GT4 I think, we could select the time record we wanted to delete, for a specific track in time-attack. That way, we could keep our best lap record with specific cars, and not having the lap record board full of laps with the Escudo. I would want that simple to implement feature back in GT6.