I've said this before, but the logistics of using the B58 in a TNGA-L / N platform just doesn't seem to make sense to me. The engine is made somewhere far away from Japan (Germany? Austria?) and would have to be shipped in, which seems contrary to the Toyota Production System which has an emphasis on JIT inventory and local suppliers. Is Lexus going to open a new local facility to make the B58 solely for the IS? This seems extremely unlikely.
On top of this hurdle, I'm pretty sure that TNGA was designed to work optimally with other TNGA components, which is why the transmissions and engines are all lumped under the framework. A long inline 6 engine, made by a different manufacturer for a different architecture, doesn't sound like something that would slot right into this framework.
From what I can tell, TNGA is designed to help reduce design choices at every step of the way - pick a wheelbase, ride height, etc and the choices get narrowed down more and more. At every step of the way, you know that a certain level of testing and validation has already been done, and you won't end up with something that is totally unfeasible, or sacrifices too much of a feature (ie. interior space, NVH, etc). Where does the B58 figure into this? It's not a TNGA-designed component, I'm not so sure you can just slap it in, hook it up to the transmission and call it a day.