MacBook Pro 2017 Geekbench Before and After Spectre and Meltdown Patch
Industry testing is/has already been done and will continue as this issue is flushed out.   According to a just released press release directly from Intel, Industry Testing Shows Recently Released Security Updates Not Impacting Performance in Real-World Deployments, the following OEMs/Manufactures have stated the following:


  Apple: “Our testing with public benchmarks has shown that the changes
  in the December 2017 updates resulted in no measurable reduction in
  the performance of macOS and iOS as measured by the GeekBench 4
  benchmark, or in common Web browsing benchmarks such as Speedometer,
  JetStream, and ARES-6.”
  
  Microsoft: “The majority of Azure customers should not see a noticeable performance impact with this update. We’ve worked to
  optimize the CPU and disk I/O path and are not seeing noticeable
  performance impact after the fix has been applied.”
  
  Amazon: “We have not observed meaningful performance impact for the overwhelming majority of EC2 workloads.”
  
  Google: “On most of our workloads, including our cloud infrastructure, we see negligible impact on performance.”


Apple also goes on to further expand on the testing stating that:


  Our current testing indicates that the upcoming Safari mitigations
  will have no measurable impact on the Speedometer and ARES-6 tests and
  an impact of less than 2.5% on the JetStream benchmark.