Welcome to the Lexus family. My 2007 RX 350 has pulled to the right since it was new. Now has over 40,000 km on it. As in your experience, the dealer told me it was sensitive Lexus steering :( After alignment, switching tires from side to side, it still was not acceptable. So, I wrote an e-mail to Lexus Canada complaining - ended up getting their servive rep to drive it. He acknowledged the pulling and approved replacement of the power steering pump and rack. Replaced Japanese rack with North American one - supposedly lighter steering touch - more assist. After all that, it was improved - did not feel like the steering was not 'centered' any more, but still wanted to head for the ditch on normal highway driving. Drove it for a while and finally after 1 year warranty was expired, another dealer did another alignment at no charge while I was paying for a replacement windshield after 3 major cracks - but that's another story. It was improved a bit more, but still takes pressure to hold the lane. Seems to be very sensitive to road camber - left lane tends to hold straight, right lane drifts right.
Apparently, the design limits the alignment adjustment they can do - cannot do camber, I think - only caster so it is hard to really do a good job of getting things lined up like they should be.
I'm planning on trying a non-dealer specialty shop to try to get it more comfortable to drive. Sorry to have to advise of ongoing troubles - suggest you not accept the dealer pacification and get them to aggressively make it right. Try driving other vehicles on dealer's lot to see if they are different.
God luck
Wilson