Yaris under load?

[CompanyXPaladin]

I'm planning a five thousand mile road trip with some friends for June. There will be 3-4 of us, and luggage for a week. Space won't be an issue, I've had more in my car for shorter trips, however, my car has the rear lowered about 1.5 inches on Eibach springs, and my Yaris seems to have an issue with more than about 500 pounds distributed evenly. The handling, acceleration, and braking suffer, as you would expect, and the rear sits very low to the ground. (I should really stop using the Yaris as a pickup truck... but it just does the job sooo well.)

I'm planning on putting load-balancing equipment in the back, and installing a front strut brace. I most likely will also install a short ram intake. I would rather not replace my rear springs - I like the way the car looks lowered a bit, and I have no idea where my stock springs have gone. Are there any other mods I should consider? Does anyone have any experience making their Yaris handle a load better? Should I be concerned about strain on the engine, with that much weight for that length of time? Thanks in advance for your advice!