Given the sensitivity of brakes to mechanical advantage, I wonder whether there's an element almost of chance, e.g. in the up-down adjustment needed for the pads on a particular frame? The Shimano brakes on my Audax bike are completely brilliant, every bit as good as the V brakes on another bike. Other dual pivots that I have are very acceptable, but not the same - even the Ultegra ones on my road bike.
This is certainly true. The MA will decrease as you move the pads down the slots. If you wanted to you could work it all out by measuring from the pivots to the pads and from the pivots to the cable anchors (this will also show you why old school centre pulls can be good). Deeper drop brakes have longer arms to compensate so if you're in the position where you can have short brakes at the bottom of the adjustment or standard brakes at the top then the latter will be more powerful (but use more cable pull). I have a bike which takes caliper brakes, mg's and 35mm tyres. it's deep enough to take long dp's or cp's with the pads at the top of the adjustment. The brakes are good but will probably need more cable adjustment as the pads wear.
The other factors that have as much, and potentially more, impact are cabling, pads and arm flex. I once bought a bike with no name (I think I know the brand from googling the part nos. on the calipers) 57mm drop brakes. They were rubbish. I changed the pads - still rubbish. I changed and optimised the cabling, lined outers, stainless polished inners, silicone grease, ground ends, good bends, etc. Still rubbish. If you applied a gorilla like grip to the brakes you could see the calipers flex - marginal but noticeable. I changed the calipers to some Tektro ones and had some perfectly effective brakes. It seems to me that caliper flex will use much of the cable force into overcoming the internal friction in deforming the caliper and not into pressure at the rim.
IME all the big 3 and Tektro brakes are fine. Some brands I've never used and there is one brand I would never buy. Calipers that have decent pivot bearings especially ball bearings are better. A lot is down to how they're installed, the pads used and where in the adjustment slot they are.
Having said all that, Shimano's NSSLR levers with other calipers (and vice versa) may change everything (for the worse). My only experience of this is using NSSLR levers with 73mm calipers with the pads at the top of the slots which as per the above seems like a good combination.
