I found the same struggles in the Grand Alpine Challenge Bush Trip.
I found the CubCrafters XCub severely lacking to deal with the altitudes. I found even at 9 or 10 thousand feet, the plane struggles to climb 100 feet/second. I have had to do many corkscrews to gain altitude, and often had to abandon the recommended flight plan in order to find a path through the mountains.
I think I will extract the flight plan and flight notes and try this bush trip with another plane, perhaps the Cessna 172 or the Beechcraft Bonanza G36.
Perhaps others who have tried this can share their clues?