The first U.S. owned team in the sport since 1986, Haas bring a car built in Italy by chassis maker Dallara with Ferrari engines and technical support.
"If all the stars are in the right place, it will be realistic but they need to be all lined up," Italian-born American Steiner told reporters at the paddock at Albert Park on Thursday.
"After the test, because the second week we wanted to do more set-up work and we haven't done it, I'm a little bit -- I wouldn't say scared -- but a little bit conservative that we hit the right set-up.
"I think the car is the right platform but now we have to find the right set-up.
"If we hit the set-up right and we can get more confidence then I think it's possible."
Guenther said the team had been working day and night for the past three months to build camaraderie but there had been precious little time for bonding outside the garage.
He said Grosjean's experience had proved invaluable for a race crew with a sizeable proportion of Formula One rookies.
"He goes in the car and when he says 'the car is doing this, I like this', we know that he can be a reference point from last year's Lotus which wasn't a bad machine," he said of Grosjean's previous team, now reborn as Renault.
"There's not one thing or one point of data that he brings to the car, it's just a feeling which he brings to the car.
"If you have got new drivers who come into Formula One they tell you what I want to hear because they don't know any better. The best (for them) is to make the boss happy."
Formula One has introduced a number of changes for the new championship, with a revamped qualifying and a clampdown on radio communications, putting more of a burden on drivers to manage their races.
Steiner said he hoped the new changes might play into the debutant team's hands as the established outfits grappled to change from the old ways.
"I think for us everything will be new anyway so I think we have got the better chance," he said.
"I'm not afraid of it. It's something new and people don't like change."