You Can Trial iOS 8.4.1
For the moment, Apple is still signing off on the iOS 8.4 update which means that you test out the iOS 8.4.1 update.
Apple's still signing which means that the loophole that allows iPhone 6 users to drop back down to iOS 8.4 is open. This means that you can download iOS 8.4.1, try it out, and move back down to iOS 8.4 if you don't get the performance you want.
Now remember, Apple will likely stop signing off on the update within the next week and a half or so so you'll need to get this done quickly. Once Apple stops signing, they will be no way to drop back down to the iOS 8.4 update.