It does not retard timing at high rpms, just at high loads with open throttle. Once you reach cruise speed, even if it is at high RPM, and are no longer at a high throttle opening, the vacuum returns and the timing advances beyond the mechanical limit. This does not apply to racing, as then you are mostly under load and WOT, not cruising. Boats are happy with full mechanical because they are, in effect always going uphill.
