I would tend to contradict. The Spi height and area are the key parameters for nose-diving, the main adds only heeling moment, because it flies in the downwind of the spi. If you sail down wind with the main alone, than yes the boat would probably be prone to nose diving. However if you have a spi you would use it, even if it is blowing.
So said, if you have a spi which is not larger and heigher than that of an F16, then you are not less safe than an ordinary F16.
It is at least my expierence (I sail with an A-cat mast and sail plus spi).

Cheers,

Klaus