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I looked up an old study by Stanley Plagenhoeff on measurements he carried out in respect to grip firmness at impact for the serve. He investigated the relationship between ball speed, racket head velocity and striking mass (depends on grip firmness). Racket head linear velocity before and after impact was measured. He saw that, for example, Gonzales hit one serve 2 miles per hour than another serve, with a swing 11 miles per hour slower. He concluded that a too high racket velocity makes it difficult to hold the racket firmly, so nothing is gained by it. He added that concentrating too much on on greater racket velocity can also result in lack of control. So you have to hold the racket loosely when serving so as not to tense up and block yourself, but, at the moment of impact, hold it tightly to achieve a high striking mass.
