Through a lot of trial and error, research, and forum help; it looks like iOS 7 isn't very compatible with Contract Killer:Sniper and iOS 8 is compatible.
I have used the iExployer program to restore the CKS1 database file from one iOS 7 device to another iOS 7.
Can anyone confirm whether a game data transfer works from iOS 7 to iOS 8?
BTW; According to ARS Technica "If you have an iPhone 4S or iPad 2, you are still going to radically degrade performance if you upgrade from iOS 7 to iOS 8.1.1. So don’t do it unless you have a seriously good reason to."
Another possibility of why game issues occur are the hardware differences between iOS 7 and iOS 8 compatible devices. I have noticed that more issues occur when more people are playing the game (end of the PVP tournaments). However, hardware should only effect the game running more smoothly versus choppy and delayed graphics/animation. Unless, the game servers some how adjust to require more available resources from our individual devices, when the game server resources become low? I've never heard of a gaming network adjusting to it's users in that way, but maybe GLU development team/IT can answer that question???
iPhone 4, usually.