Thanks for asking this slippery question… on all our minds I think. Probably you do need to do a major cull now, but I wonder if a smaller and therefore faster filled, warmer heap would help. My heaps have been steadily warm, not massively hot, since April and I have noticed no more slugs in them than elsewhere. Maybe consider an enclosed heap like a plastic beehive, although they can become soggy unless enough ‘brown’ is added, such as chooped stems and woody bits, scrumpled or shredded paper etc.
A final, calmer thought: although as you say slugs are breeding now, and autumn will be tricky, if we have a cold snap as in December 2010 their numbers will be much reduced: so a lot of slugs now does not necessarily mean a lot next spring.