One reason the tentative 400' height limit is stupid is that Tysons needs to be legitimately interesting, and from a skyline perspective, that means skyscrapers. Rosslyn is nice and dense but nothing can be iconic when all the top buildings are within 20-30' of each other in terms of height (although it will certainly be quite dense by 2015 with Central Place + 1812 N Moore done).
They should gradually auction off rights to construct ~10-20 towers north of 400', as several in the 600-800' range would do well to accent the monotony (maybe a 1,000'er as well
).
One perspective where the skyline is already somewhat impressive is coming into Tysons on Route 123, especially at night.
I do know that the tallest in anyone's proposal for their Tysons land was SAIC's tower, which will be ~430' with the roof element. The others;
392 ft: Capital One
~380 ft: NV Commercial, Cityline (several), Georgelas
~350 ft: Tysons II (several, some taller...), Tysons I (several)
Sure I'm missing some, but that's roughly 10 buildings significantly taller than anything in Tysons today, yet none are tall enough to truly stand out... (although the SAIC tower will have the highest elevation, so it could actually make an impact...).