Earlier quoted context omitted.
That seems to depend on the task. The highest end CPU available, even the 15", is the i7-8650U which is a 1.9Ghz @ 15W part. This is also how they pull off 17hr battery life, they are putting an ultrabook processor in a 15" laptop. Current 15" MBP can be had with a i7-7920HQ, 3.1Ghz @ 45W which I would expect to be quite a bit faster for multi-core workloads.
Which they run at 20W
i7-8650U is rated at 2.1Ghz @ 25W in it's TDP up config, so running it at 20W may get you 2.0Ghz instead of 1.9Ghz. But this doesn't tell the entire story either. Modern processors are constantly adjusting their frequencies across all cores to hit their power targets. e.g. if the thermal management can't sustain 20W in perpetuity in may actually run slower than it's rated speed. Unfortunately this isn't something that is easy to determine from most benchmarking tools. Things like geekbench will only tell you the system could maintain certain performance for the length of the test, which is usually only a couple minutes, you may find that doing longer running work will cause throttling.
If you really want to know how it'll perform under heavy workloads, you probably want to hit it with prime95 or blender for 30+ minutes and monitor core frequencies to see where it ends up running at.