Norris and Piastri took second and fourth respectively in the recent British Grand Prix.
It was a breakthrough result for McLaren and its upgrade package after struggling through the first half of the season, but it was also a big boost for rookie Piastri, who took his best result in F1 by some margin.
The highly rated Australian has repaid the faith shown in him by McLaren when it went to great lengths to prise him away from Alpine last summer, and according to team principal Stella the 2021 Formula 2 champion has also had a positive impact on Norris.
"Oscar's performance is just outstanding if you think he's a rookie, Stella said. "And right away, even in practice, he was immediately quick.
"This allows also some synergies between the drivers because both can see where you can improve from each other. And this elevates the performance of both drivers.
"So, not only we are impressed, but there's a net benefit in having two competitive drivers because both can benefit."
After a year on the sidelines as Alpine reserve driver, Piastri's race craft was identified as a potential area of improvement.
But the 22-year-old's mature race craft was on full display in Silverstone, where he kept Mercedes' George Russell behind and only lost a potential maiden podium due to an unfortunately timed safety car.
I think the speed that Oscar is able to deploy we have seen it already. The element that comes out [in Silverstone] is that his race craft," Stella added.
"His ability to keep gripping the tyres, clean driving in an intense racing situation, thinking about the safety car restart on a hard tyre, it was immediately there.
"This is a mature race craft, which is quite impressive in a guy that is so young."
Piastri was driving in Silverstone with the radical upgrade package that Norris debuted in Austria, but without the new front wing that only the Briton received in Silverstone.
McLaren is set to bring the Australian's car up to the same spec as his team-mate in Hungary, with more updates due to arrive on the MCL60 to complete a mid-season overhaul that improved its downforce levels and efficiency.
