I have not kept up, I know there are some shared platforms, but I thought Sienna had a dedicated one, being much heavier. I could see Highlander and Camry, maybe. Although I think it's Highlander and Sienna share, while Camry and Avalon share, and Corolla and RAV4 share? don't feel like looking, since I'm not shopping.
No disagreement about dropping $15k into a Camry being a waste of money. OP wanted to drop $500 or less. IMO it's not worth that either, but it's not same order of magnitude. And he just wanted more power in the hills, did not ask anything about carving canyons or repeated 60-0 stops.
When I souped up my VW I was already happy with its handling--and that was on 195/65R15's. I had no illusions about its cornering abilities--nor did I ever test them out. Same for braking (hey I got over 100k, nearly 200k out of the OEM brake pads). But the 100hp PD, while "nice", was way nicer after 40 more hp.

I thoroughly enjoyed that extra power on my commute. Today, I would love to have that again, my current Camry is as boring as it gets--yet again, its cornering is just fine for my needs (braking, that is lacking, but probably not worth addressing). Just underpowered in getting up to speed.