performance effects of ignition timing advance

Just curious. What exactly are the performance effects of advancing the ignition timing? I always set it to 5-6 degrees BTDC when I have to take the car for a smog check but thereafter reset to 8-10 BTDC (just because I once read the slight advance is good). Not sure if I've really noticed any difference in mileage and power, although I have noticed increased knocking in hot weather when climbing steep grades with the 8-10 degreee advance.

What would you Metro folks advise as the optimum timing and what performance effects should one expect at that timing?

Mike
95 Metro 3 cyl, 5sp, 93,000 miles, 47 mpg and running like a top