Does anyone know what would be an optimal worker's setting, of course, based on worker's "capability"?
General advice, set your minimum difficulty to your hashrate in GH/s divided by 1.4. Info on difficulty, work submits per minute and the easy mode perk:
https://bitcointalk.org/index.php?topic=27062.msg3142713#msg3142713Than, what's Stratum extranonce2?
The stratum extranonce2 setting is useful for subpools. Don't worry about it. Most people have no use for it.