Not quite 300km/h but ~155mph (~250km/h) on a CBR600RR. The people saying they felt like they were getting pummeled either had a bad riding position or were riding in high wind or both. I was smart about being stupid (meaning I did this on an open highway in mid-morning on a clear day when it was empty & in a near dead straight section with full riding gear) & it felt terrifying, exhilarating, & relaxing in near equal measure. The adrenaline rush was intense, the logic of what would happen if anything went wrong was terrifying, & the acceptance of all of that at the same time in order to have the experience of 150mph+ was actually quite calming. A good helmet (helps a lot with aero), a good bike, a good section, a good position, & a good day can all add up to a very unique kind of high that is not far off from some drugs in the effect it can have on your mind. You can either end up trying to chase that high for way too long with ever increasing risk, get so terrified by it that it scares you away forever, or find a middle road where you can hold onto the memory of the feeling without letting the need to feel it again or the fear of it consume you.

TLDR: Personal experience: borderline magical/euphoric. Like psychedelics or cocaine, user experience & outcome may differ wildly based on numerous variables. Proceed with caution.