The German squad were third strongest in 2007, but they had a troubled start to winter testing with their F1.08 after opting to take a more aggressive approach in order to get closer to the top two teams.
But despite their problems, BMW have been very competitive in the first two races of the season, finishing second both in Australia and Malaysia.
Theissen says the team's goal is still to score their maiden win this year, and he reckons their risky approach is already paying off.
"Our goal is to close the gap to the leaders and win our first race," said Theissen. "That's a fairly ambitious goal and we're sticking to it. In the first two races we managed to open up the two-way battle.
"We were faster than one of the two top teams on each occasion. But one shouldn't draw any premature conclusions.
"We're still not on a par with Ferrari and McLaren-Mercedes. However, the latest results are certainly a great incentive for everyone. The clear message to the workforce is: we can make it."
He added: "We've still got several arrows in our quiver and certainly haven't exhausted our entire potential yet. The next major step is planned for the European curtain-raiser in Barcelona.
"For us one thing is clear: risking a major concept leap in the winter has paid off. Now it's up to us to systematically continue along the path we've embarked on."
Theissen also praised his team after they kept their cool following the early problems.
"It was the first real acid test for our young team," he said. "Since the first outing in Valencia the day after the launch, we refused to be diverted at any stage and systematically continued along our planned path.
"Measured data and driver feedback were compared, causes analysed and a whole package of measures adopted - with success.
"In the weeks following the launch of the F1.08, we not only brought the car up to speed but also learnt a great deal in the process. In retrospect these were extremely productive and motivating weeks.
"(Technical director) Willy Rampf and the entire team deserve a huge compliment for never once losing sight of the goal even under extreme pressure."
autosport.com