The 918 Spyder is better than P1 and LaFerrari on the most racetracks. Salomondrin made some nice comparisons with a professional race-driver. The 918 Spyder beats P1, LaFerrari, Viper ACR Mk V and Veyron Super Sport and also the direct comparison with the P1 and the Viper ACR Mk V.
Normally the P1 and LaFerrari got better race-tires on, so the 918 Spyder got a clear disadvantage there. Motor Trend and EVO both tested the 918 Spyder vs. the P1 and the 918 Spyder won both comparisons when both cars drove on the same tires (Trofeo R). Beside of that Ferrari was always so afraid to make some official tests because they know that the 918 Spyder and the P1 are faster on tracks.
All 3 hypercars are amazing pieces of engineering but the 918 Spyder got the best performance on the most tracks, the most comfort-features and technical stuff like assistance-systems, multimedia (3D-soundsystem), navigation-system with two multi-touch displays and with it's AWD and 4-wheel-steering you can drive it 365 days per year! Porsche really know how to build perfect super- and hypercars. The 918 Spyder, 991/.2 Turbo (S) and 991 GT3 RS are currently the best cars in their segments.