Having worked in Soho with Piccadilly being my main station for 5 years and Marble Arch for a decade before, and Bond St prior to that, I can assure you popular restaurants are still busy. I tend not to go away in August and work through and find that its mainly those with children who need to take time off and it is mainly the 2 weeks leading up to the public holiday at the end of the month.

Restaurants are pretty full still because smart tourists have read tripadvisor et al and know what the trendy places to be are and have booked them out. Restaurants that may be quieter will be in the City as they don't have the benefit of the same passing tourist numbers. Those bordering Shoreditch may be ok as Summer activities do drive locals and tourists there alike.