Yeah, it's too late now, Apple have stopped signing installs for iOS7 on devices that are compatible with iOS8. I ran iOS7 on my iPhone 4 and it wasn't too bad. It was sluggish in places, but more than usable. If you watch some of the performance videos on YouTube you might be able to get a better idea.
As a side note, I have a few iPad 2 devices at work which has similar specs and they are running iOS8 without any noticeable performance issues.