The new XK convertible is Jaguar's best opportunity to add some excitement to its slow-selling lineup. Premiering at this month's Detroit auto show, the cabrio will go on sale in April-at the same time as the coupe. The convertible, which typically comprises the vast majority of XK sales in the United States, shares the coupe's 300-hp, 4.2-literV-8, paddle-shifted six-speed manu-matic, and aluminum construction. The XK uses a traditional canvas top-rather than a retractable hard top-but in the down position, the folded roof hides neatly beneath a hinged aluminum panel. Expect to pay about $80,000.