let's say this can pass the brain barrier.
evolution has created a brain where the production of neurotransmitters is highly regulated, which is why we can eat meat, if every tyrosine molecule got converted to dopamine in the brain we'd self-destruct. so if there is a pathway that converts this compound to dopamine, why would you think it would make a difference? it would be highly regulated and you have to get the dopamine to the right place in the brain.

that's why such small amounts of dopamine agonists work, milligrams compared to the need for almost a .5 to 1gram of levodopa, the body makes it tough for some amino acids to even get into the blood stream.
the brain doesn't regulate agonists but they stimulate dopamine receptors, that's why l-dopa helps but not tyrosine, the brain regulates dopamine, if there is too much it is broken down or sequestered, if not enough the brain makes more, assuming you don't have parkinson's.