bits of useful info here

grumpyvette · Mar 1, 2009

http://home.mindspring.com/~jim_fisk/id1.html

http://www.hotrod.com/techarticles/engi ... index.html

http://www.gmhightechperformance.com/te ... index.html

http://www.turbominivan.com/tech/turbo101.htm

http://www.holley.com/data/Products/Tec ... 0244-1.pdf
yes your supercharged engine DOES tend to show a slightly richer plug color and fuel/air ratio meters do tend to indicate your effective mixture is a bit richer for the exact reason you stated.
thats one reason super charger cams tend to have a wide 112-116 LSA, so that theres a bit less overlap time where compressed boost could enter and partly exit into the exhaust port while both valves were open

btw I ran across this picture of a TPI turbo set-up