I long to know if using "--alt-preset standard --athlower 9" has any bad side effect (exept increasing the bitrate).

With other (read: not crippled) lossy audio formats, lowering the ATH by a constant value will never harm quality.

With mp3, you must consider the case where an "--alt-preset standard" frame is already near 320kbps. In that case, because of that stupid hard imit, the encoder had to choose a tradeoff. In the tradeoff, "--alt-preset standard" has maintained as much quality as possible.

In your case, lowering the ATH by 9dB brings an additional requirement which will be a burden for the encoder, and will skew its judgement, thus making an unbalanced tradeoff and lowering quality. Such frame will top off at 320kbps and quality will be lower.

Of course, if your song never (or rarely) hits 320kbps, there should be no problems.

YMMV. IMHO to be safe you should use --alt-preset extreme or insane.