Yeah, your CPU should still be very capable for gaming, so a CPU upgrade wouldn't make much difference to game performance in most titles. A graphics card upgrade would likely boost frame rates a lot more, particularly if you are running recent games at maximum graphics settings, or at a resolution higher than 1080p.
Are you considering getting a new card or a used card though? If buying new, the 1080 Ti is a previous-generation card at this point, and its prices are typically no longer very competitive compared to the newer cards when buying new. The RTX 2080 generally offers slightly better performance than 1080 Ti, so a 1080 Ti probably isn't worth considering unless it is priced lower. And of course, if buying used, you will likely be getting less or no warranty coverage, so keep that in mind.