Will this upgrade guarantee a performance boost?

Going from a 6600k to 8700k (with Z370 board similar to current Z170)

I'm asking because there is a little time during which I can still cancel the order. If gaming, the only thing I use my PC for, will see no notable performance increases then I'd rather cancel and wait.

We would need more information, mainly resolution/monitor, what GPU and what games

2560x1080 at 120 or 144hz (75 or 100 for demanding games; monitor is freesync, gpu is a 1070, so no magic tear eliminating options for me)

Many games, including upcoming titles like FFXV and Far Cry 5. Right now though I really need Kingdom Come to run more consistently.

Kingdom Come will definitely benefit based on what I've seen. As more and more games get multi-threaded, more cores/threads are going to help. You're essentially tripling the number of threads by upgrading, and multi-threaded games will benefit.

Is the upgrade worth the cost? That's up to you. Could you just go with a used 7700k (~$240) and keep your motherboard? Another option.

Spend that money on a gsync monitor, you'll be much happier.