...in judging Darcy to not be speaking straightforwardly when he makes his "energetic" comment, but I'd take it one step further--rather than condescending, I believe he is actually teasing Lizzy! He's trying to be funny, and therefore "fires" his second "shot" in their "merry war".

I also think Lizzy's responding "shot" is a masterpiece of subtlety.

From the start, in disguise of speaking to Charlotte, she is already talking to Darcy, when she chastises Charlotte for "always wanting me to play and sing before anybody and everybody!" Lizzy is remembering that in their last encounter, when it was Bingley who was touting Lizzy to Darcy, and so Lizzy wants Darcy not to leap to any conclusions that this is a pattern that is arising because Lizzy wants this, i.e., that she is so hard up for a boyfriend that she needs her friends to promote her.

And, in passing, isn't "strange creature by way of a friend" a virtuosic turn of phrase, which is on a par with the best repartee in JA's model, Much Ado About Nothing.

But then Lizzy betrays that what Darcy thinks is important to her, because she makes sure Darcy knows she does not claim to be a great player. Why? Because she realizes that he probably has sophisticated musical taste, and she doesn't want him to think she lacks the taste to realize her own artistic limitations. She cares what he thinks in spite of herself!

So then, when Charlotte prods her again (and I totally agree, this is a scene that shows exactly why Lizzy and Charlotte are such close friends, they enjoy a level of subtle raillery and teasing that I Lizzy cannot get even with Jane, who is just a tad too serious for such game-playing), Lizzy then gives Darcy the zinger which, I think, she has been planning since the assembly, waiting for the best moment to sock it to him.

"There is a fine old saying, which everybody here is of course familiar with -- 'Keep your breath to cool your porridge' -- and I shall keep mine to swell my song."