The 20 top senior executives and senior technologists at Mercury Computer Systems Inc. in Chelmsford, Mass., have been driving around town in brand-new silver Porsche Boxsters. President and CEO Jay Bertelli had promised to lease for them the $48,000 Porsches if the company doubled its stock price during 1999 and held that price for 30 days. The stock tripled and then split toward year-end. At the end of the two-year leases -- which are in the individual employees' names, the employees have the option of purchasing the cars. Bertelli also leased two other Boxsters in the company name that will be loaned for two weeks at a time to high performers among the company's 450 employees. A 23rd Boxster has been ordered, and, at the end of Mercury's fiscal year on July 1, one of firm's top performers will be chosen, at random, to drive that car for a year.