The FL270 restriction at GIBSO is a relic of previous airspace sectorisation; it goes way back to when the descent point was 30 miles before SAM The current sectorisation could accommodate FL300 or similar.....an econ profile is probably low 3 hundreds. Precedent....FL140 at BEDEK, but no-one expects to regularly cross NUMPO at FL270 even though the distance is about the same.
I believe that FL270 by GIBSO has many advantages for the HURN sector by allowing sequence/speed/integration with other traffic. An aircraft presented hot and high with a tailwind can be virtually uncontrollable.
My pprune buddies are correct when they state that a new clearance replaces a previous clearance with a proviso that any previous restriction must be repeated if it still applies. A changing traffic situation may not require re-application of the restriction.....or it may be evident that it will/has been complied with....or an individual co-ordination by the controller with other ATC sectors/units has been completed. [Many restrictions between sectors are called standing agreements and are put in place to allow vast amounts of traffic to pass through the system without individual co-ordination; when it is quiet there is more flexibility]