Perfect time alignment between drivers is not possible without the use of digital filters, even if the drivers are "supposedly" aligned using a sloped baffle. Phase changes with frequency so while one can achieve perfect alignment at a specific frequency at a specific point in space, it is impossible to achieve this throughout the entire bandwidth without very complex digital filtering.
With the Sierra-2 (and all of our products) - we employ phase shifts caused by the crossover filters to achieve very tight phase integration and time alignment. I would not recommend tilting the cabinets backward as this would throw this off.
hope this helps!