I just purchased a Prius Prime to upgrade my 2014 Plug In Prius. In the 2014, I had installed the TRD Sway Bar and a front strut tower brace, and these add-ons made a significant difference in the handling characteristics. Are there any OEM or aftermarket upgrades folks can recommend?
I have no experience with them, but the catalog of TRD functional parts for the Prius PHV lists the Motion Control Beam (MS303-47003) and Member Brace Set (MS300-47003). The installation instructions (in Japanese) are linked from the price list page.