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LEAP (Lightweight Exo-Atmspheric Projectile) is the kill vehicle on SM-3, but otherwise you're correct -- SM-2 Block IVA was the "lower-tier" supplement to the SM-3 "upper tier" interceptor. Together they would have covered the whole flight envelope, roughly analogous to Patriot PAC-3 and THAAD.

The difference is that SM-3 is a pure exo-atmospheric interceptor and is optimized for the role. THAAD, on the other hand, has a mix of low exo-atmopspheric and high endo-atmospheric capability. That sounds good on paper, but forced a lot of design compromises, like having to cool the seeker window against atmospheric heating, at the cost of weight and seeker complexity. The result is that SM-3 has a wider engagement envelope than THAAD, especially versus intermediate-range threats -- it can take them on both ascent and descent, while THAAD is a pure descent-phase interceptor.

Check slide 6 in this briefing (less than 1 MB):

http://www.ndia.org/Content/ContentGroups/Divisions1/Missile_Defense/Mr.%20Altwegg's%20Presentation.pdf

Also factor in the potential of SM-3 Block II, which blows up the second and third stages to the full 21-inch diameter of the Mk 72 booster and takes full advantage of the volume available in the Mk41 launcher (the so-called Full-Caliber Standard). That's going to give SM-3 a huge kinematic advantage over the much smaller THAAD.