Yes, 240% will outscale 150% AD scaling--especially 240% total versus 150% bonus.
To be precise, the "base" damage of Rek'sai's E is effectively 270 true damage at level 18.
By comparison, Darius' R has a base of 680 true damage.
The difference is 410 damage, and Rek'sai has +90% scaling more than Darius. The point where R beats D in true damage is >455 bonus AD, which improbably high. It is possible.
That said, what's your point? Almost every ability that any champion has can be compared to a similar ability, and one of them will have higher scaling than the other, which means an eventual point of dealing greater damage with the same AD/AP. This is a pretty normal thing in League of Legends.
I'm simply curious, why does this matter?