ビザの延長申請中に国外に出ると、申請しなおさなければならないそうです。
May I leave Canada before my request for an extension has been finalized?
Yes. However, if you leave temporarily and you do not have your new document before you seek to re-enter Canada, you will have to re-apply (either at the port of entry if you have the right to do so, or at a visa office outside Canada) and pay another processing fee.
http://www.cic.gc.ca/english/pdf/kits/guides/5551E.PDF入国できるかどうかは入国管理官の裁量なのでなんともいえませんが、別室で質問されることくらいはあるかもしれません。