In the era im familiar with (ps3, 360) everyone used the first 9 coefficients. You can read the original Ramamoorthi paper for better theory applied to lighting

In the era im familiar with (ps3, 360) everyone used the first 9 coefficients. You can read the original Ramamoorthi paper for better theory applied to lighting. But yes it’s an approximation. If you have a ton of terms it looks like a bitmap like you said.