Really the only reason most people do it is for keeping an intercooler core clean. It will keep the upper end clean as well, but you won't notice a benefit in the butt dyno.
Yes, this is true.
I lose about 1 mpg with it blocked, don't believe it is doing any harm, don't think it improves performance too much (it must lower the intake temp a little cause the exhaust gases are 500*+F) and it keeps the intake tract and TB licky clean.
Sure the combustion temps must lower a hair cause the intake charge is being diluted with sooty exhaust gas instead of heat...and power...produced by air/fuel mix. It is added after the MAF sensor.
The whole idea is too re-burn a little fuel rich sooty exhaust gas that hasn't combusted fully, for emmissions purposes.