the 1/2" diameter loss of the 3" collector will limit you far more than the 1/8th inch of primary size, the collector will be like a Peak Torque RPM cork and limityour torque peak to a lower RPM, whereas the primary will slowly pull down the power (relative to the larger pipe) at whatever that RPM peak is achieved.

I whacked the 3" collector of a set of Hooker 1 7/8" at the back of the primary tubes (shortening them a bit too) and slid on and welded a 3 1/2", And when I went from a RB standard port to a Stage 6 headed low deck with the raised ports I never even had to take the collector flanges off the X pipe! fit perfectly.