aterpster, discontinuing the RNAV GPS approach isn't an emergency IMHO but a maneuver based on accuracy's degradation prediction called RAIM, that doesn't mean you've lost the accuracy yet it's a prediction. We're sliding off into a complete separate topic here cover by the FAA as well. In order to be eligible to fly this kinda approaches you must be an approved operator using approved equipment and certified staff etc. Long story short you must check the availability of GPS and file a none GPS alternate. Prior to start the approach somewhere 2 NM prior to IAF the RAIM function of your on board equipment once again checks the integrity of the whole mission and if you're given no warnings you're good to go. The chances for you to lose GPS Primary are nearly NIL and even if you do like you described you still have sufficient accuracy to conduct missed approach. Of course it'd be easier to simply tune a VOR but why bother in the age of GPS. BTW a modern jet easily makes up to 11% climb gradient in normal conditions. No afterburner needed.
Aircraft Performance Database V2.0 - Details 