https://code.wildfiregames.com/D5202
This patch is committed for a27, and changes the immortals quite a bit. Rather than reverting them to standard champions like those that many civs have, they are cheaper, weaker, and retain their weapon switching. This allows the Persian player to field a lot more of the immortals.
I also removed their speed penalty, which was a little excessive, and slightly reduced their switch time to 6 seconds from 8.
I could make this into a community mod patch if you like. It would be worthwhile for testing anyway.