A friend of mine recently asked me a question that I was uncertain of, so I'll try it here with all the brains.

If rider A and B rides up a climb in exactly the same time, and with the exact same weather conditions, will they have the same Watt/kg? If rider A weighs 70 kg, and rider B 80, and rider a rides with a powermeter. At the top rider A has had an average wattage of say 300 watts. This equals to 4,28 Watts/kg. Would you be able to take rider B and say 4,28 x 80 = 342 watts?

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I like Spanish bikes and cars. Riding an Orbea Orca and a Seat Ibiza.

No, there are many factors that influence the time/speed, but all being equal: There should be a surtain grade were the are the same. I just dont know what that grade is.

http://www.training4cyclists.com/how-much-time-does-extra-weight-cost-on-alpe-dhuez/

Here's a fun little experiment showing how much weight and rolling resistance will affect performance.

Apparently, on Alpe d'Huez, 1,8 kg of extra weight equals 1.54 min time difference.
More interestingly; riding it with a tire pressure of only 3 bars is just 1 min slower.

To answer your question --> It depends on the gradient.
Very steep: Time depends almost entirely on relative power (W/kg)

Interesting question, but I suspect power is non-linear.

Geoff is correct, but with a bunch of assumptions, you're not far off. I came up with 338.59W using the following equation (p=kg*m^2/s^3). I made an excel sheet for my wife and I when we only had one PM to do the same thing, and the results seemed reasonable and later confirmed w/i small %.

For your inputs, and my output, the following assumptions apply:
-bikes and gear weigh the same
-rolling resistance is the same.
-Cd is the same or that the speed is so slow it doesn't contribute.
-Riders were side-by-side over the climb.