Ok...


The SPI loads up the mast above the hounds and diamond, so if the mast is going to break it will be the unsupported section just above the hounds. so we can rule out the diamonds as being a factor in supporting the mast.

the SPI is loading up the mast in a forward and leeward direction. you would hope more forward if the boat is going fast. The main support for the forward force is the main sheet, a tight main sheet will act as a back stay preventing the mast from failing from the forward forces.

The leeward force is being supported by the major axis of the mast, particularly in a wing mast as it is very strong across its major axis (the fore and aft one). if you don't have the mast rotated you will be presenting the minor axis of the mast to the leeward forces and make it more susceptible to failure.

as far as performance goes, the mast should be rotated downhill to 90 degrees anyway.