A lengthened COAX cable will not affect wifi performance.
Depending on how far you want to move the modem, Shaw can provide you with a 6ft cable free of charge. Or atleast they used too.
If you choose to buy a custom length of cable, stick with quality brands and avoid dollar store specials. Low quality cables can affect the reliability of the modem. Also, avoid splicing or splitting any of the COAX. This will also reduce reliability.
