The Mercedes Formula 1 test driver, who finished third in the FR3.5 title race with ISR last year, was brought in for last week's test to help the team's development ahead of this season.
Bird, who was battling sickness during the test, was in the top five in three of the four sessions over the two days. And he believes that ISR now has a car more capable of winning the title than it did last year.
"The team has done some good work," Bird told AUTOSPORT. "The engineers have really dug deep and they've found something which could be crucial for their season.
"Obviously their drivers will now have to do the job. They've got to use this car, because it is a step forwards.
"If I'd had the car from this test at the end of last season, then I believe we would have won the championship. That is how much it has come on."
Bird also praised the team's revamped structure behind the scenes, and thanked ISR for giving him a chance to get on track.
"I'm impressed with the strengthened technical team," he said. "I think we can expect very good things from ISR this season.
"When I was on track we were consistently near the front, despite not always focusing on single-lap pace.
"It was a pleasure to team up with ISR again and I'd like to say thanks to [team boss] Igor Salaquarda for entrusting me with this test."
ISR only has one driver signed for this year - Sergey Sirotkin - and as it stands Bird is understood not to be a contender for the second seat.
Salaquarda added: "We are still negotiating with three drivers about this year's contract for the second car, and I believe we will have a strong and balanced pair of drivers.
"I would like to thank Sam for helping us, his involvement was of great benefit.
"Compared to last year we made a step forward. I am very satisfied with the test results and how the team works."
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