To my understanding, there's two differences between OT / Premium and all other trims:
- Thicker laminated windows to reject outside noise and improve thermal insulation
- Extra door seals which further reject wind noise - GXs with these seals have visible black rubber between the front & rear doors (also between the rear door and C pillar) when the doors are closed, which OT and Premium (non-plus) trims don't have
If you're looking for sound insulation, I would bet that you'd get more value by strategically applying extra sound deadening under the carpet and in the doors. I'm thinking a product like dynamat.
IMO the GX isn't a particularly quiet cabin even in Lux+ trim - it's shaped like a brick no matter what. For the cost of a set of four doors, you can get your car professionally insulated at a pretty high end shop and cut down road noise significantly more than by adding some double-laminated windows and a few extra wind seals. Plus it should significantly improve the sound quality of the factory speakers. Only downside is it'll probably add a few hundred pounds of extra weight.