Nico Rosberg dominated Thursday practice on the fabled streets, and has locked out pole position since Bahrain a month ago.
But in the races, the W04 burns through the Pirelli tyres at an alarming rate.
"In Barcelona we were 70 seconds off in the race, which is one hell of a big chunk, and really difficult to make up in two weeks," said German Rosberg on Thursday.
The problem could be less dramatic in unique Monaco, however, while chairman Niki Lauda told Brazil's O Estado de S.Paulo: "There is no denying that we are making progress.
"We still have a lot to do, but we have good indications that we have evolved in that direction."
Ferrari's Felipe Massa told La Gazzetta dello Sport: "Mercedes will be hard to beat in qualifying.
"I also saw a long run from Rosberg and it was very fast. They don't have so much tyre degradation."
Red Bull's Sebastian Vettel admitted he is hoping for some Saturday rain.
"Otherwise it will be hard to be right at the front. Mercedes looks very, very strong," he told Bild newspaper.