Based on my 750 hours of 912ULS experience in my RV-12, I offer the following non-professional observations:
-- It seems unlikely to me that over-weight floats would cause a sudden total engine stoppage.
-- At high-power settings, my engine does not appear to be significantly impaired by over-weight floats. I suspect that the high-power fuel flow through the carbs somewhat masks the poorly performing over-weight floats.
-- My first sign of over-weight floats is typically a whiff of fuel smell during climb-out, slight roughness at low-power settings (rough idle), and some signs of fuel residue at the end of the carb vent lines.
-- I suspect that you might have experienced some vapor lock/cavitation combined with slightly old summer/winter fuel.