My number 5 WR is a high catching reciever but slower than the rest of my other 4 WR's. I was gonna use him in certain plays as a TE. Is the penalty extreme or is it worthwhile ?
You can put your WR in any slot. The only thing to consider is the out of position penalty (oopp). The WR, even if he is blazing fast will be nerfed and not nearly as quick / fast if you simply had him in the WR slot.
Best thing to do if you are trying to get a mis match from the TE spot is find a burner TE who is more of a receiving TE. They are out there if you are willing to recruit.