A delegation of Jewish leaders expressed concern to President Reagan yesterday about the improved range and firepower that the administration recently agreed to provide for U.S. jets previously sold to Saudi Arabia. Reagan assured them that he would not permit the action toupset the balance of militry power in the Middle East.

"If we had our druthers, we'd prefer there not be a sale," said Gordon Zacks of Columbus, Ohio, a member of the board of governors of the American Israel Public Affairs Committee. "But we were reassured and [Are] comfortable that the strategic balance will be maintained and that Israel, from a qualitative, military security point of view, will emerge stronger than she is going in."

Zacks and Max Fisher of Detroit were the spokesmen for 32 Jewish leaders who supported Reagan's candidacy last year. Their talks with the president and high administration officials covered a range of subjects, but the group's chief conccern was Reagan's decision to sell the Saudis equipment to improve the range of 62 F15s they purchased from the United States.

Israel fears the jets could be used against them. As a means of allying Israeli concerns that improvement of the jets is "destabilizing," the administration has agreed to provide Israel with an additional $600 million in arms sales credits.

Fisher said he was relieved by the president's response on the issue but indicted that his feelings were not shared by everyone.

"Some of us felt better; some of us didn't feel quite so good," he told reporters after the meeting.

Zack said the Jewish leaders wanted a "quid-pro-quo movement from the Saudis on political questions that would be supportive of further stabilizing the area."