The only potential in-game benefits will be due to DirectX 12. If your game isn't specifically coded to take advantage of DX12, your games will gain nothing.
You can always get the free upgrade, and after your system is finished upgrading, your Windows 10 license for that machine will be registered with Microsoft's activation servers. If you don't like the upgrade, you can roll back fairly painlessly for the first 30 days, or reinstall Windows 7 on your own after that. I've seen a fair share of failed and botched upgrades of Windows 10, so back up anything that needs backing up before proceeding.