..is it possible replacement upipe (over oe) -could reduce power?

 

It's plausible I guess. Ideally, I reckon, you would want the manifold collector outlet / up-pipe inlet and up-pipe outlet / turbo inlets to be perfectly matched (even if the up-pipe is tapered).

 

The diameter of the up-pipe (and indeed of the entire exhaust manifold) has a major effect on the way the turbo spools and develops power. The larger the diameter, the lower the gas velocity will be, but the less restriction there will be at high engine speeds.

So yes, there certainly is a potential for an up-pipe that's too big to result in a loss in low-end response, and, depending on how much boost you run, there's also potential for a pipe that's too big to cause a net reduction in power output.