The Brackley based team has utterly dominated the opening four 'flyaway' grands prix of the new turbo V6 era.
But the start of the European season is traditionally the scene of vast improvements up and down the pitlane, as teams add the first major developments to their fledgling cars.
"Until now we have had a big gap to our nearest competitors," championship leader Rosberg is quoted by German-language Speed Week.
"But three weeks have passed since China and for sure a lot has happened. So we will have to wait and see," said the German ahead of this weekend's Spanish grand prix.
"I hope that we still have an advantage, but we will have to see how big it is. I am confident that we will still be the fastest.
"But we are also aware that the competition has been working around the clock to catch up," added Rosberg.
At the same time, Barcelona is usually a key indicator of how the rest of a F1 season will look.
Not only is the Circuit de Catalunya a frequent test venue for the sport, the nature of its layout is a good indicator of how a car will perform on most types of tracks.
"A car that performs well in Barcelona should perform well everywhere," agreed Paul Hembery, tyre supplier Pirelli's F1 boss.