Replace the plugs with those recommended by NGK for your car. Use NGK or Nippon-Densa, however, and not an off-brand. These are the only two companies that Toyota uses as original equipment suppliers, so you know they’ll have the exact replacement. Put something like Champions in and you’re sure to have problems…I speak from experience.
No. you’re ont likely to see an increase in mpg. If your plugs were badly worn enough for that to happen you’d not be getting those mileage figures in that car and you’d probably have see a CEL by now.