I was on the same search, and though not concluded (I still have to try blacks and clears), I must say I'm somewhat pleased with the NMB RT-6856TW RD boards. The basic specs are:
Keyswitch Travel: 0.140±0.020 inch (3.56± 0.5 mm)
Travel to Make: 0.090±0.020 inch (2.29± 0.5 mm)
Operating Force: Momentary Action - 2.0 oz. nominal (55 grams)
Feel: Tactile.
Life: Meets or exceeds 20 million cycles.
My measurements - distance from top of travel to important reference points:
0.050" (1.3 mm) to tactile bump, 55g
0.098" (2.5 mm) to activate, 40g
0.130" (3.3 mm) to bottom pad, 50g
0.145" (3.7 mm) to end of travel, ~100g
Hard to believe, but it's a RD board that makes before it bottoms out, with a soft landing due to the RD design. It a damn nice RD, although the keys are a bit wobbly (but not as bad a ALPS). The wobble gives them a little clack sound when you release them, but not too loud. I use one at work and it's quieter then the Das brown, Razer BWU blue, or IBM-M.
The left shift key measured 55g as well, even when hit off center. Way better than the 65g I was getting with the Das or the 80g I was getting with the Razer BWU.
The space bar is stiffer due to an extra spring under it, which I believe you can remove. It does have the typical NMB nasty sharp edge to the space bar though.
Overall I can type faster on this than anything else I own, with very little bottoming and then very lightly and cushioned due to the dome.
You can get a NIB one right now here:
http://cgi.ebay.com/NMB-Cyrix-OEM-PS2-Keyboard-/220701468954?pt=PCA_Mice_Trackballs&hash=item3362d5291aJust something else to consider, although not mechanical.