It is likely to impact on system performance, which is why it isn't enabled by default. The problem is the way utorrent works, as it creates hundreds of little files that it then creates the final downloaded file and these have to be scanned.
If there is a function in utorrent to have the AV scan the completed download then this is the path and file to enter, C:\Program Files\Alwil Software\Avast4\ashQuick.exe.
So there you have it, if your system is relatively fast then you may not notice any impact in having the P2P Shield scan it, give it a try and see.
