Alright, this is going to open another can of worms because I'm not sure how execution cap handles multiple cores. But on the plus side it looks like all your benchmark has to do is run bitcoin core with -reindex and then measure the time it takes to finish from debug.log and also using stuff like top to keep track of resource usage. But automatic profiling with systat where the metrics are stored in other log files is better IMO.

According to https://www.virtualbox.org/manual/ch03.html#settings-processor, the policy limits the cap for each virtual core, so you could have for example 2 cores both running at 50%. I think that to accurately measure Bitcoin Core performance, we must also measure how much the multithreaded verification speeds things up so it would make sense to be able to do things like emulating a Core2 or i3 with enough vCPUs as threads it supports, and then use the execution cap to try to emulate their baseline processor speed. This, in my opinion, will make more useful results than just testing at 10%, 20%, 30% etc. caps.