Core 7900X has 4.5 GHz single core turbo clock. 7700k single core turbo is 4.2 GHz. 7900X has 300 MHz clock advantage in single threaded applications. It also has larger L2 and L3 cache and higher TDP headroom. It will beat 7700K in single threaded applications. This is the first time a high core count Intel HEDT chip will beat their 4-core mainstream flagship. It is a big deal for enthusiasts who don't want to sacrifice application & gaming performance.
Anandtech at least seems to believe that 4x larger L2 cache has a big impact on IPC:
"double of cache size decreases the miss rate by the sqrt(2), and is liable for a 3-5% IPC uplift in a regular workflow. Thus here’s a conundrum for you: if the L2 has a factor 2 better hit rate, leading to an 8-13% IPC increase, it’s not the same performance as Skylake-S."
But it is all speculation now, since we don't yet know the associativity and latency of the new 4x larger L2 cache. It both stay the same, then we see a noticeable IPC gain in general purpose code. Also it's not yet known how much the new L3 cache decreases performance. Skylake-X L3 is a smaller non-inclusive cache after all. My guess is that the new cache system is generally faster, but there could be some applications where the performance is actually slower.