According to some miners here old firmware of s9(november firmware) is a better firmware than the new firmware but its recommended to don't reflash the firmware if your s9 is fine, just use the firmware when you having a program or firmware issue.
Never had s9i but I think they are both the same.
I found a modded antminer s9 firmware here
https://bitcointalk.org/index.php?topic=4513567.0 and according to the thread this firmware can improve the hashing rate of s9 but expect for more power and higher temp of the miner and also it could avoid your miners warranty so if you are not sure to use this firmware and your miners is running fine then don't change anything and stay with the current firmware.
About s9i you can try the method here to overclock the miner but this guide is for s9 but you can try to apply it on s9i here
https://bitcointalk.org/index.php?topic=3239820.0 and it can be also useful on your s9 autotune firmware without reflashing your firmware.
