LOL. RCF chassis is much stiffer than the IS. IS and GS chassis are soft and not rigid enough, if I were to be perfectly objective. Good or bad, the flex a lot under high torsional stress situations like hard turning. RCF has no noticeable flex and I have seen it almost go tripod on steep approach angles. That is why all of that effort went into making RCF have all of the constructed as a unibody construction using laser screw welding rather than the traditional welding method. It has braces and rigidity beams in the doors , underbody, trunk, in the hood and the sills for added rigidity that the IS or the GS platform does not have. Anyone could tell that by looking at the size of the doorsills and the weight of the doors because of the rigidity beams running through the door. The handling numbers also agree with me.
Frankenstein formula is used by all of the leading brands. Porsche uses it, BMW uses it. Look up how 1M was built. Lexus is not the first. It is not as optimal as building a brand new chassis like they did for the LC, but it is the next best thing they could do. You are showing your lack of knowledge based on what Matt Farrah said. That chassis wins races and used in all of Lexus racing. I can post interview of the chief engineer's interview explaining why the 3IS chassis was not sufficient for their RCF racing homologation aspirations and why they needed to build a new chassis. They even were testing the RCF chassis in the ISF CCSR guise in racing. They could have made a 2 door coupe easily out of 3IS chassis for much cheaper, if it was better for racing.
I don't doubt IS500 F-Sport would be a phenomenal car. With a narrower axle width/track and less mechanical grip with the same amount of power, it would be easier to break the rear loose making it a hooligan's car. Some people might finding it more engaging, but it is back to the purpose of the two cars.
If you dlslike the RCF, that is your choice and I would not say anything about that. However, when you trash a car based on viral reviews then you are going to get the facts thrown back at you.
If you are thinking IS500 F-Sport is going to outhandle the RCF without the suspension/chassis hardware RCF has, that is your own subjective opinion.