I don't understand WHY people still compare coupes to a convertible. The 918 Spyder may not be a traditional convertible where the roof is removed automatically, but ANYTHING with a removable roof (such as a targa) is STILL a convertible.
Being a convertible, it is fair to say that the 918 loses in straight line performance. Even the LP700-4 Aventador Coupe catches up with it past 155mph (250km/h) and will eventually beat it.
Same exact thing can be said about the SLR, Enzo, and Murciélago LP580 Coupe versus the Carrera GT. Yes, the LP580 and the Carrera GT have the same 0-300km/h (186mph) time.
Its (the Carrera GT's) MOST fair rival is NOT the Enzo, but rather the MC12 from sister brand Maserati. Plus, of course, the SLR McLaren Roadster.
The 918 DOES NOT directly compete with LaFerrari, the body style is different and convertibles (especially soft-tops) create more drag than coupes and hardtops.
The LaFerrari Aperta is the 918's most fair rival. McLaren didn't make ANYTHING to rival the 918, but the 675LT Spider or 720S Spider is pretty close.
The LP670-4 SV was only available in coupe format, so the LP650 Roadster is the Carrera GT's closest competitor from Lambo.
The LP750-4 SV Roadster is the 918's closest Lambo competitor and maybe EVEN its closest competitor because of AWD.