Hi Frank,
As slime-close-parens-at-point is now deprecated I am in search for a direct replacement. I had that function bound to ")" so the exact number of right parentheses got inserted at point.
I know it won't answer your question, but have you already considered to use paredit[1] ? I installed it a few months ago and I no longer face unbalanced parenthesis problems in any lisp mode.
Xavier