Nice car! Lunar Mist Metallic too, just like my gen 5.5

. I think going to 16" would be nice and honestly, the 1MZ is a strong motor and I doubt you'll feel much of a difference in power due to extra weight or whatever else.
I'm looking at staying with 15" too but that's because I just bought expensive new Michelins (LOVE them) which I don't want to waste. I'm thinking of going with OEM 15" rims if I can find them or waiting until I need new tires and purchasing 16-17" rims + tires at the same time.
Keep us updated and welcome to TN!