Officially, the fly rate given in a Fly spell is the maximum rate of flight. Pace multipliers do not and cannot be applied. However, a steep dive might be able to increase speed by actually letting gravity control the speed rather than the spell (i.e. approach terminal velocity), but that is a decision that I would leave up to the GM to make (and decide actual dive speed).
Flight that is natural or that is caused by a talent would be different. (note: flight via a talent MIGHT be like the spell -- it depends on the wording of the talent and the ability itself).
