Richard Jemison has done this, Alfar7 here on the BB. My restoration engine work uses them on both sides. Ideally you would want to replace the steel seats with #954 bronze seats with Ti valves for heat transfer, but on a recent performance vintage race head, I left the good steel seats in place. It still works fine. I don't know if Alfar7 replaces the seats with his Ti valves.
My question would be one of cost vs value on the return side. If you are restoring a valuable head and wish to reduce valve mass but maintain strength, or go to a thinner stem, the Ti valve might be worth while. In racing engines, the additional strength might make it worth while at high rpm. With unusually high exhaust temperatures, for what ever reason (Turbo?) there might be a pay back, but then you want the bronze seats as well. The will damp bounce to a slight degree, and seal well for track use, but this can get pretty expensive.
You need a reason, and the best ones are still racing applications and preservation/restoration applications.
This is just my opinion from my experience. Alfar7 may have other opinions.
Todays quality stainless steel valves are excellent for most applications, and quite inexpensive by comparison.
It wasn't that long ago Alfa's used sodium cooled three piece exhaust valves in various engines. They were expensive, and spectacular when they failed. They were relatively heavy, but did provide excellent exhaust valve cooling and heat transfer ... for a while....
the stem size will be 8 mm... i am useing tc2 valves, these are 102 mm in lenght, so i can use vw tappets and shims..brd racing makes these valves for racing..i will be useing 45 intakes, and 38.5 exhaust vavle