If you're running even both SD boxes at the same time, you shouldn't see a decrease in internet speed.. I'm usually running 1 HD box and 1 SD box, and I still don't see a drop in my speed.
Also, which boxes do you have? If you have the old RCA (big white) boxes, get them swapped out ASAP. The new SA boxes are much better.
I've had Telus TV for almost 2 years now, and have gone through quite a few growing pains with them, but I still recommend it over Shaw any day.