Brendon Hartley thinks McLaren dumping Honda at the end of last year was "a mistake".
"I think they’ve made a mistake," the New Zealander told Marca sports newspaper on Monday.
"The engine is very driveable and I have more power than last year," he added.
Hartley had just tested Toro Rosso’s newly Honda-powered 2018 car in Barcelona, and team boss Franz Tost also sounded positive.
"Honda developed at the end of last year and the trend is still the same," he said.
McLaren endured an abysmal three years with Honda and have switched to customer Renault power for 2018.
But Tost thinks Toro Rosso can target the top of the midfield this year with Honda.
"We have big medium and long term plans with Honda," he said.
"I always say the first three positions are gone in the championship, but everything is possible in the fight for fourth place."
As for the new Honda relationship, Tost said everything is going well.
"We are pleased with the progress in reliability and performance," he said.
"Working with Honda opens up access to vast resources and we are already benefitting from that."
On the other side of the coin, McLaren had a high-profile mishap as its 2018 winter season began when a wheel fell off Fernando Alonso’s orange car on Monday.
"I’m happy for you," the Spaniard laughed to reporters. "It’s a good story."
Indeed, he said it was only a minor issue and that everything is going well with the car and new Renault engine.
As for Tost saying he predicts a trouble-free season with Honda, McLaren executive Zak Brown smiled: "I don’t think there’s a team in the pitlane that will not have some trouble.
"Looking back over the past few years, a problem-free year is virtually impossible.
"We wish Honda and Toro Rosso all the very, but we’re focusing on our programme. And we’re very satisfied with our progress," Brown added.
He also applauded the FIA’s clarification that engine suppliers need to provide their customers with identical equipment, even in the area of software.
"We have always been calm about Renault giving its customers equality," Brown said. "It was never a concern, but I’m glad that the FIA has clarified it."