Pretty neat upgrade. I don't really understand why this is possible without additional fees, but glad to hear it is working anyway.
Basically, when you first enroll coins in to Whirlpool, it does so with what it thinks will be a safe fee for relatively quick confirmation of a 5-input-5-output coinjoin. If, when those coins actually come to be coinjoined, it turns out that fee is higher than necessary, it will enroll additional inputs in order to lower the overall fee rate back to a more appropriate level.
So that is what the Sparrow's Premix Fee Rate is supposed to do, right? Things that make the postmix-ed UTXOs able to be remixed repeatedly and the coinjoin that has more than 5-input-5-output is come from that premix fee, if the current pool participant/condition has a low fee that would reduce the number of inputs and postmix remix rounds availability. Am I understanding it correctly?