Installing smaller injectors to reduce consumption.
 

Anybody ever tried to use injectors that have less flow to get better mpg's ?

either A. it'll run lean, the O2 will compensate, and no change. or B it'll run leaner than the O2 can compensate for which could damage the engine. like running a turbo without adding bigger injectors.

I was told that a lot of cars run slightly rich from the factory. WOuldnt this help.?

you were told mostly wrong

most cars are tuned on the rich side at WOT

The way to do it is run smaller injectors preferably with a more efficient design (4 pintle or something) at higher pressure. Then you get better atomisation, for the same fuel, this may translate to better burn and the ECU will actually lean them back a little.