Five years after becoming one of the first commercial banking companies to buy a healthy thrift, a western Pennsylvania company has seen its experiment in shifting a thrift's product mix bear fruit. Since 1990, when First Western Bancorp purchased First Federal Savings and Loan of Western Pennsylvania and changed its name to First Western Bank, the profits of the thrift subsidiary have nearly tripled, while its return on assets has more than doubled. In 1994, the subsidiary earned $4.3 million, up 186% from 1990. Return on assets was 1.03%, up 151% from 1990. "That shows you the impact of changing the balance sheet mix from thrift products to traditional banking products," said Thomas J. O'Shane, president and chief executive of First Western Bancorp. The thrift's numbers also contributed to the parent company's own strong performance. New Castle-based First Western, which also has a separate bank subsidiary, earned $15.2 million for 1994.