I have 2 ti bikes which are both (I am quite sure) all 3/2.5. Both TiCycles customs. We talked briefly of 6/4 for both but it seemed hardly with it. After 17,000 miles, I do not regret that at all. In fact, I never think about it. Both are stiff. 59 and 58 cm. I weight 150. For me, very stiff, in fact the stiffest bikes I've owned. I have zero need for more. I have also ridden an early Merlin which, though perfect for me, was not stiff. It's all about the tube diameters and details. TiCycles has been known for making stiff ti frames for years.

Ben