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In theory, titanium should be more flexible since it's a softer material than aluminium or steel (please correct me if I'm wrong)
You're wrong :)
Oh, you wanted me to expand on that? First, hardness is not the relevant metric, although in many cases greater hardness correlates with the property you do want, which is elastic strain limit. When you open up a clamp, you strain the inside surface most, and you need to keep that strain within the elastic limit of the material. The amount of strain for a given amount of bendage depends on the wall thickness, with thinner being better. In that regard, bicycle grade Ti and steel generally win over aluminium, because they are stronger so people use less of them. They also have a higher elastic limit than common aluminium alloys, which is why they are used for springs and aluminium alloys aren't. The TL;DR of all this is that you can open up Ti and steel clamps more than aluminium ones, and you can probably open Ti ones even more than steel ones given the alloys and tempers likely to be used on bicycles. The bigger question is why you're even considering the purchase of a Ti stem in the first place, it's a poor choice for all the same reasons as it's a poor choice for frame tubes.
gbj_tester-
Thanks for the elaborate answer, very interesting! There's a couple of reasons why I would like a Titanium finishing kit and I guess none of them will seem rational to you which is only fair since I wouldn't argue that it's a rational choice :) The main argument is that I like rare and unusual bits as a collector and design enthusiast. Second, I would like the bike to be fairly light, which is not justifiable in rational terms since it's a steel tarck bike and won't be used for hill climbs or anything like that. If weight was the primary objective I'd build a seta. Third, the frame is rather stiff (lugged columbus max with shorter ST than I usually ride) and I read that titanium seatposts have shock absorbing characteristics which in theory should make the bike a bit more comfortable. And with a TI seatpost I want a matching stem although I can see that for a stem you would want maximum stiffness especially for out of the saddle sprinting. In all honesty, most of this is just wankery and at the end of the day the vast majority of us buy bikes and bits for subjective reasons because we like them or because we are #buyers and not neccessarily because we need them for performance gains or for functionality. TL, DR: I want them because I want them :)
Sig_Arlecchino
Getting compact bars in quill stems can be a major PITA but the penny trick definitely helps. I was wondering whether I can do the penny trick on titanium stems as well or whether there is a higher risk that they crack. In theory, titanium should be more flexible since it's a softer material than aluminium or steel (please correct me if I'm wrong). Would be nice to know before I buy a ti stem since I definitely want to use compact bars.