You link to the documentation of an old postgres version (9.2) that is not supported anymore (I know that google often puts these old links to the documentation at the top -- annoying).
But in this case there is an important distinction: recent postgres has default implementations of sp-gist. So the 'limitations' when implementing your own sp-gist class, while still true, need not worry us here. You will see that newer docs have a page "Built-in Operator Classes". My example uses the built-in sp-gist class. The 'limitations' are just warnings for those who implement their own sp-gist class.
It would be interesting to have an example dataset to compare the performance of any forthcoming pure-perl solution with what postgres indexing can do. Or graphic libraries (like you mentioned earlier).