I'm not too familiar with all this nifty "power-weight calculators" and all the charts and all that, so I used good old fashioned physics.
For a 18% 1 km climb (cycling distance) you travel about 920 meters horizontally and 165 meters vertically. Assuming Euro is 67 kilos, plus another 6 kilos for his bike, that means, to climb this 1 km he must put out around 120 kJ of work. At 10 km/s this climb would take 6 minutes. That's 360 seconds. And since one Watt is a Joule/second, that's right around 330 watts. That's also neglecting friction, both friction in the drivetrain (I believe a free-hub bicycle is about 98 or 99% efficient), and friction due to road surface. There's also wind resistance, which only Euro will know. However, I think it's safe to assume that we're pretty close to 400 watts. 400 watts at 67 kg is about 5.9 watts/kg.
First of all - all your caculations are done by this calculator already.
http://www.kreuzotter.de/english/espeed.htm
Which I linked to in my earlier post. With the addition of wind resistance, rolling resistance and drive train ineficiency put it at 400 watts, so you were pretty close.
In my earlier post I also mentioned my approx speed.
67 kilos, 10km/h and cadence of 36
BUT - the thing that Plin and Slvoid did was a schoolboy error - I didn't say how long the 18% gradient is, as you mentioned. All I said was that I could ride up that gradient in that gear. As it happens, the 18% section on this particular climb is probably about 100 meters long.
This morning I did the same section in a slightly lower gear 39/21, having spun through the lower section in 39/26.
Regarding my potential category - I ride with people who are (I guess) equivalent to US Cat 1, and I'm as quick as them up hills. I'm not as quick as them on the flat, though. It would/will take me a couple of years of dedicated training and racing to get up to that level in terms of points, but it has been pointed out to me that I have potential to ride at that standard.