That's a good question. I'm pretty sure it's a rate limited conversion process. I would search PubMed or PubChem for articles on the subject and see if anyone can get them. It shouldn't be hard.
I'm guessing that tyrosine supplementation does very little for central DA levels. TH is rate limited, and oral administration without using carbidopa to block peripheral metabolism will probably mean little or none is getting in the brain. I'd have to look into 5HTP more, but you're capable of doing this on your own.
I'm not aware of any notable side effects associated with even high dose tyrosine supplementation. That's not the case with L-Dopa therapy, though..