The tallest tire I've ran was a 185/65/14. I saw an improvement over the 195/50/15 - probably because of the width. Yes, it bumps up the effective final drive, but only minor changes were necessary in driving technique. My car has Integra brakes so original tires are not an option.
The wideband O2 is a permanent mount.
The factory wideband is very stable, nothing short of impressive. The air/fuel usually stays within one point while cruising, but it really depends on the conditions whether it runs in the low 22's or high 23's - not sure why.
Perhaps this will answer your other questions -
http://ecomodder.com/forum/showthrea...nda-23288.html