Rogers Pass is a 6% grade. Kicking Horse Pass & the Coquihalla are both 8%. If you use those routes. I have seen many TV's & other RV's on the side of the road, spewing steam. My 2 cents, these are steep mountain roads.
More info here:
http://www.th.gov.bc.ca/popular-topics/faq.htm.
On the other hand, the "most dangerous road in Canada" - Hiway #1 around Golden, is now almost completely rebuilt & MUCH more friendly to both man & beast.
Don't forget it's not just about getting to the top, you have to go down the other side too. Spend as much time & effort on checking the brakes as you do checking the cooling system. Keep right, keep your flashers on, & your speed down & all will be well. You will be in line with all the pro's who drive the big rigs & know how to handle this road. Have fun & enjoy your adventure. If you are going to more than one NP, consider the pass for the whole Parks system. It might be cheaper for your party.
-Steve