Welcome to the club!
If you want fuel-efficient, I would start by considering adding the XV Crosstrek Hybrid's front bumper w/ underspoiler and rear spoiler extension to improve MPG. Also, adopting the Yokohama Geolandar G91 P225/55R17 tires and aerodynamic wheels that are standard on the XV Crosstrek Hybrid may also yield some additional MPG improvement.
As for performance boost, depends on what you want for performance. If you want higher engine output, options are extremely limited. For suspension, there's a lot more options, including stiffer bushings and even the option to swap your non-hybrid suspension for the "sport tuned" stiffer hybrid suspension not found on the regular gasoline-only models. And if you're willing to burn off more money, there are actually a lot of options available for stiffening the chassis, including a solid front strut tower brace from SSD and various stiffening products from Cusco.
"Why would I want to profess my feelings for some 3D girl?" - Tomoya Aki from the anime Saekano