Right now I am using an Onkyo TX-NR900 (110wpc x 7) to run Polk RTi12 (mains), CSi5 (center), and FXi3 (surrounds). I am pretty happy with the sound but I know that the RTi12's can handle a lot more and will really sing with more power.

I was thinking of getting 2 M200's and using them to bi-amp the 12's since this is possible with Polks. I would use the Onkyo to run the tweets and mids (1 x 1" tweeter and 2 x 5.25" mids per speaker) and the M200's to run the woofers (3 x 7" per speaker).

What do you guys think? Will this improve to overall sound and would it be enough for me to hear the improvement?

Thanks

You might try driving the Polks with the M200's alone, without any help from the Onkyo - the Polks are somewhat power-hungry speakers, but 200W seems to be comfortably sufficient. Another forum member tried driving some similar Polks with the Model 7100 amp (7x100W) but moved to the Model 770 (7x200W) and was very pleased by the performance with the 770.

...and trying to (passive) bi-amp speakers with a receiver and amplifiers with no level controls would most likely give very poor results.