Essentially yes, even with optimal power mode turned on the GPU will still go up to it's turbo clocks when put under load. Performance in games should not be affected when using optimal power mode for the vast majority of circumstances.
I'm not sure what the point of high performance mode is. It's useful if for some reason you wanted to force base clocks at all times, cant think of a good reason to do that off the top of my head but I am sure there is one.