I have 3 Ti bikes - none with a Ti stem. Funny thing is I have a Ti stem and don't know what to do with it - I guess i'll have to build up a Ti bike to match it
You cant assume ti stems will all feel the same in the same way that ti frames by different brands feel different. I've had ibis, moots & seven titanium stems & they all felt slightly different. none of them felt flexy. the dog-bone cook bros stem is a flexy stem in torsion.
Tti stem on ti bike, but, if possible all other parts in ti (seatpost, bar…).
For the stem, choose a moots : a bit heavy for a ti stem, but the stiffer !
I've got a full ti bike (even the crank and chainring !) : Only pleasure with it