Only thing I could find was a comparison of a Power8 server with an x86. On that the Power8 was a little under 2.5 x as fast as an equivalent (24 core) x86 machine from HP. The x86 was an "Ivy Bridge" generation processor.
I would not be surprised to find that the new Power9 processor was around the same, around 2.5 x faster than the equivalent x86 server - and at something between 3 x to 5 x the price. I've long stopped using Power architecture, it was a great combination for serious workloads using Power and AIX, but one gets very tired of paying through the nose for it. One of the final straws was paying something like 5 x the cost for an extra network card in a Power machine. For a test we put it in an x86 machine and it worked showing it was essentially identical only almost exactly 5 x the price.
Way back, say 20-25 years ago IBM had the possibility of having a real shot at dominating the low to mid-range server market with AIX and Power. Compared to the then relatively new Windows NT and x86 it was more powerful, more stable, and had better power user features. But to get seriously into the market they needed to compete on price as well, which would mean either chopping the prices for their highly profitable i line or differentiating by pricing essentially identical hardware at two very different prices between the AIX and i lines.
IBM chose to keep their profits flowing from captive i customers and niche market their AIX/Linux machines. They are now a legacy competitor with almost insignificant market share. They may n ot have been able to replace Windows and Intel, but they never gave themselves a chance. Back in the late 1990's, the Power/AIX or Linux server was a very viable potential competitor, but the chance was missed.