And I don't see how conquest of Sicily, or even Malta, could have change the state of seapower in Mediterranea. Any expedition to Turkey or Egypt, with Royal Navy on the heel, and fierce muslim opposition, would have been non-sense after Trafalgar.

The British thinking on this (Nelson, Pitt, others) was that Sicily was essential to holding Malta. If the French acquired Sicily, Malta was sure to fall. If the French control both Malta and Sicily, they then control the Med. The French fleet at Toulon was still there and very much a threat. Without Sicily and Malta, holding the French in Toulon becomes difficult, mainly due to supply issues. So, while you may believe that the French were no potential threat to Royal Navy in the Med., that's certainly not what the Royal Navy or the ministers in London believed.