Leclerc set the final day's best lap of one minute and 30.322 seconds on the fastest C5 tyres with Mercedes' George Russell a mere 0.046 slower on the C4 tyres.
Sauber's Guanyu Zhou was third fastest, also on the C4s, with Red Bull's triple champion Max Verstappen fourth and 0.433 off the pace.
The Dutch driver, who won a record 19 races last year, was using the slower C3 tyres that will be used in next week's season-opener at Sakhir, however.
"Overall it was very good today. We focused on the long-running to get more of an understanding of the car and it was good to see the performance of the car was very strong," said Verstappen.
"We made the plan to not focus on pure lap time and tried different compounds, so I am happy with the laps that we did today."
The final day of running with the new cars was halted for some 80 minutes by a dislodged drain cover at Turn 11 -- the second such incident in two days.
The track action resumed after repairs with the usual lunch break dropped and the session running through to the evening under floodlights.
Leclerc's car suffered some floor damage in Thursday's incident and Friday's stoppage was triggered by Red Bull's Sergio Perez.
Ferrari's Carlos Sainz had been fastest before the suspension, with the Spaniard also topping the timesheets on Thursday with the best time of testing as a whole - a 1:29.921 effort on C4 tyres.
Ferrari said they had completed the equivalent of more than seven grand prix distances, 423 laps equating to 2,257km.
"The car is reacting in the way that we wanted it to, but it is still difficult to assess its level of competitiveness right now. The feeling is better than the base we started from last year," said Leclerc.
Despite Ferrari showing good pace, and much improved tyre wear compared to last year, Red Bull continued to impress rivals with their RB20 car.
"I think Red Bull clearly are out in the distance," said Mercedes' seven-time world champion Lewis Hamilton, with others agreeing.
Mercedes said they had also focused on long-term performance rather than outright speed, covering 1,952km in total over the three days.
"Compared to last year's car, the feedback from the drivers is very different and more positive, which is encouraging," said the team's trackside engineering director Andrew Shovlin.
"It's never easy to make predictions on pace from testing, as all teams are quite effective at hiding their pace when they want to. It's clear that we are in a much better position than 12 months ago though."
The record 24-race season starts in Bahrain next week, with practice on Thursday and the grand prix on Saturday as Formula One squeezes in the first two rounds in the Middle East before the start of Ramadan.
