I currently have a 12-30t rear cassette which needs replacing. While I'm at it, I really want to switch to an 11 tooth to get the extra 9% in top speed. But the problem is, even the most expensive (e.g. Ultegra) component that will work with my bike only goes up to 11-28, and if I stick to Tiagra I'm stuck with 11-25. For long steep hills on an off day, it's really nice to have the 30 tooth.
So my second thought was, the bottom 2 or 3 cogs usually aren't fixed to the cassette, they are individual cogs that can come off. So maybe I could get a 12-30 replacement, and then a single 11t cog to replace the 12t or maybe I could even drop the 13t so the stepping goes 11-12-14. This would be ideal as I use the 12t and 14t heavily but I virtually never use the 13t.
Would this work?
From what I can see, I would also need an 11t lock ring. I'm having a difficult time finding both a lock ring and an 11t cog which claims to be compatible. For instance, I found one cog which claimed to only work various cassettes which were already 11-xx and which were 2-3 grades above my Tiagra components.
Update: I found this PDF file on the Tiagra CS-4600 which suggests I need part Y1YF01000 ("Lock Ring for 11T Top Gear"), and Y1Z81100N ("Sprocket Wheel 11T (Built in spacer type)"). But I can't find anywhere (for instance Amazon or Google Shopping) to buy these part numbers or the corresponding parts.