Preferences right above the "save" button in GT-Life mode, then at the top "arcade options" (however), set AI aggressivity to 10.
It probably won't affect A-spec races.
Sorry, I can't remember the correct names. I know this is not exactly what you're looking for, but I hope it helps at least a bit.
What would be fantastic is if PD allowed the same race options found in Arcade and online races (in terms of AI level and driving physics) and included it in A-Spec races as well.
It's not just about making the AI more difficult. It's about you choosing the right car for the race so you can race against the AI instead of just annihilating them with an overpowered car.