In reality testing means nothing...I remember last year when even in Q3 in Australia after the first runs we thought Mercedes and Ferrari were so close...and then came Hamilton's last run boom...half a second plus faster than Ferrari! But come the race and almost the first half of the season and Ferrari were pretty much faster! But that has to do with development and nowadays the top 3 teams at least are able to develop rapidly!
This year though Ferrari seem fastest everywhere...I've been watching F1 TV these 4 days and their car looks so nicely balanced and stable...Mercedes on the other hand to me seems even on today's low fuel runs (or low fuel we think) like having to push beyond the car's limits to get that lap time! Not to mention the softer tires to get about the same time as Ferrari on harder tires!
One more thing that looks nice for the 2019 season is that all the teams look closer than last year...even the midfield looks a bit closer to the top 3! Let's see...