Tires, diff fluid, shocks and droop will do far more for tuning and traction than a plastic C hub or spindle could in my experience.

Some flex is certainly understandable from a handling perspective, but a lot of this also comes down to personal preferences. I personally prefer stiffer arms and hubs/spindles. On asphalt and higher grip surfaces, the car is much more neutral and consistent feeling too me. Not a huge difference, but it’s there. The stiff arms are actually one of the best hop ups you can get for these cars in my opinion, but anytime you increase the stiffness of these parts, you can expect a less forgiving drive to some extent.

You can also do a simple bearing friction test with the plastic parts and aluminum and see the resistance increase with the plastic parts. Stick an axle through them both and use the same bearings them give them a spin. See what you think.

That’s been my experience anyway.