Return water in pot to a boil; add more salt. Cook spaghetti, stirring occasionally, until al dente. Spoon 1/2 cup pesto and 1/4 cup pasta cooking liquid into a large bowl. Using tongs, transfer spaghetti to bowl and toss vigorously, drizzling with oil and adding more pesto and cheese as you continue to toss, until spaghetti is glossy and well coated with sauce. Season with salt. Divide among bowls; sprinkle with cheese.