I literally am so p***ed off with iOS 9 and I can't even go back... iOS 9 backup won't work on 8.4, my old 8.3 backup won't work because Apple ain't signing 8.3 no more, I can't shift update to 8.4 and last time I done a reset all settings nothing changed. Any time I use emojis in anything the app instantly slows down and hangs for ages, the phone gets so hot when using, cpu is constant 60%+, WiFi calling is broken, and I won't even go in to the battery life. Having to charge it at least twice a day (Thank God for EE Power Bar). Its just a mess and 3 betas in and its still horrible for me. I severely hope beta 4 pulls out some miracle because this is ridiculous.
Your 8.3 backup will work with ios 8.4. You just can't use a ios 9 backup with anything lower