As a result of a continued sharp decline in its stock price, CMS Enhancements has extended the expiration date for 2.5 million subscription warrants that enable the owners to buy CMS stock at a designated price. The Tustin-based company, which makes computer storage products, said the expiration date for exercising the warrants has been extended to May 31, 1988, from Dec. 31. The company's board also reduced the price to exercise the warrants from $10.50 to $7 cash, plus seven warrants for each share of CMS common stock. The extension of the warrants is aimed at giving holders the opportunity to benefit from any rise in CMS stock in the next several months. CMS's stock closed Tuesday at $2.25 a share, up 25 cents, on the over-the-counter market. The stock traded above $5 after the Oct. 19 market collapse and has been above $11 in the last year.