yeah as above, 6 speed doesn't mean it's a 5 speed box with an extra gear above 5th. the overall spread of ratios isn't that much different to a 5 speed.
as for sitting at 3000rpm at 100kmh for hours, it isn't an issue. (and fuel usage isn't an issue either). they were designed for this. pretty much every 4 cylinder car on the market will do it. skylines aren't some mystical creature that is totally different to every other car on the market. they are just another car.
of course when you compare it to the big aussie 6's and 8's that sit on about 1800rpm at 100kmh it seems high, but they only rev to 5000-5500rpm, and have a lot more low down torque, so they can do it. doing 100kmh at 1800rpm in a small turbo 6 would be horrible. you'd be dropping down a gear or 2 at the slightest little hill, or even if driving into a headwind, lol.