I would say that for sure the GS is under appreciated, and the most underrated model in the Lexus lineup. I'm glad to see the GS F finally appearing after years of torturous wait. Also it will be nice to see the 2.0T slot into the GS as well.
I do agree that the ES is holding back the GS to some degree. I also agree there is too much redundancy that exists between the Avalon and the ES. Yet, the Avalon, ES, and the GS are all simultaneously selling well, so maybe there is a market for all of them?
To me it makes logical sense to get rid of the ES, focus more effort into the Avalon, and further focus on the GS. Unfortunately I know Toyota won't do it, because the margins on the ES are high.
I will give credit to Toyota though, they have differentiated the ES and GS this generation distinctly enough that both are selling well.
With all that said, on the other hand the ES still appeals very much to the traditional Lexus customer. Also it means something that the ES has been around since almost the brand's birth. It's an older model than the IS for example or the RX. It still maintains the slot in the Lexus lineup as the "baby LS" so to speak. Those that want a large, comfortable sedan but can't afford an LS.
So in summation, I believe from Toyota's point of view, there is enough market room to have both the Avalon, ES, and GS. Once we accept that this is how Toyota very likely views it, then all we can do is hope that Toyota puts extra effort into the GS. With the ES in the lineup, Toyota has to put more effort into the GS to make it more competitive.
I feel the GS should be the volume sedan of choice. With having an Avalon and the ES sharing the wheelbase of the current Avalon, there is too much redundancy. Keep Avalon, axe ES and GS will do a lot better.
Plus GS should be offered in more engines than just the same carryover 10 year old 2GR-FSE. If I was in product planning, the current 4th gen GS would be structured exactly like this:
GS 200t - 2.0L I-4, 8AR-FTS Turbo, 250 hp minimum (for Europe/Asia)
GS 300h - 2.5L I-4 Hybrid, 2AR-FXE, 230 hp minimum
GS 350t - 3.5L V6 Turbo, 2GR-FTE, 350 hp minimum
GS F - 5.0L V8 Turbo, 2UR-GTE - 560+ hp minimum.
I feel the GS F got shafted of its true potential. The 2UR-GSE and AAE80 were directly transplanted from the RC F to save on costs for certification in all the markets and to dodge the gas guzzler tax here in the USA.
Or perhaps a stop-gap until the next-gen engines are ready? I totally agree as well that Lexus is not realizing the true potential of the GS. I hope Akio Toyoda is aware of that.
I also don't want to dabble into far off rumors, but I can hope that something like a Lexus "FS" line is in the cards, or is being discussed. Then the regular "F" models would start to make a whole lot more sense.