Mercedes has owned the class-leading engine since the introduction of V6 power units in 2014. Ferrari fought Mercedes for the championship until the final stages of last season but has launched a stronger title challenge in 2018 and leads both the constructors' and drivers' standings after 10 races of the 21-race season.
Ferrari qualified within 0.1s of Mercedes' pole time at the British Grand Prix and managed to win the race, an event which saw customer teams Haas and Sauber show impressive pace in the midfield. Horner thinks the Italian manufacturer is now the team to beat.
"I think it's setting the benchmark now," Horner said when asked about Ferrari's engine.