There's no relationship from the usage of the P3D SDK and the overall texture quality of a scenery. They are totally unrelated.
Using the P3D SDK on KIAH won't result in a big performance gain, since we already got rid of all the slow legacy ground polygons in 2010 with KDFW. Other developers are advertising miracle-like fps increases by switching to the P3D SDK, but only because the finally got rid of such ancient methods just now, something we already did many years ago.
And, we already sort out minor rendering differences in the two sims with our script logic, which requires other developers to distribute a different set of files, but it's done automatically, since the scenery script knows under which version of the sim is running.
So, even if we might be forced (for purely marketing reasons), to recompile with the P3D SDK, it won't result in a very noticeable fps increase, because what could be optimized, already is.
About the texture quality, as discussed so many times after KIAH has been released, it was intentionally designed to be fairly light on memory, following too many complaints about CYVR supposedly "causing" OOMs. Of course, you don't just turn on a magical switch that increase the texture quality of a scenery that was designed in a certain way.
So, maybe, your question might have been "Could KIAH be REMADE for P3D4, with higher quality textures, now that OOMs are no longer an issue, since textures look dated ?", which of course it's a sure yes, everything can be remade, but I guess we have much higher priorities, like KORD for example, which in ADDITION of "looking dated" (way more than KIAH), it's even OUTDATED from an aeronautical point of view.
