Everyone is different but if you're healthy, and take some supplementation, you can still produce about 100 ngs of T while using 100-200 mgs of exogenous testosterone. It's not much, but it's SOMETHING, and libido is better not only because you have MORE T, but because the body is still getting the signal to manufacture it. It's just a more natural state.
300 mgs non stop? That may be pushing it and shut you down completely after a while.