All of Sierra's cards seem to have hibernation issues, even their express card models. When I was with Sprint, I got so tired of it, that I switched to the Novatel Merlin express card, and it works perfectly with hibernation. Basically Sierra and Novatel took different routes with the way they implement the internet connectivity. Sierra has it setup up as though your connecting to a Wi-Fi network. Novatel on the other hand installs as a ethernet connection which seems to make a world of difference. Only thing I notice is that when waking up, it takes the Novatel about 6 seconds before it reactivates, but thats not a big deal.
If you do what you propose, you may cause more issues. The card would continue to send data to the PC, and the PC may think, what the h3ll, I thought I cut power to this device already, and then the PC ends up hanging because it cannot sleep correctly. I would just get rid of the Sierra card, if its fairly new, I would take it back and threaten to leave Sprint if they do not give you a proper hibernating card like the Novatel. Also dont mention that its for a car pc, because they will make up $h!+ so you cant trade the card. The reviews say the Novatel isn't that good with internet, but upon testing, it worked better then the Sierra card with things like internet radio, signal strength, basically anything that doesn't use a internet browser, worked better on the Novatel. When it comes to internet browsing the Novatel is weird. Sierra right away starts to load bits up, where as the Novatel seems to wait till most of the bits are downloaded before it displays on the browser. So the Sierra is steady loading the page, but the Novatel seems to hesitate, but the hesitation is only like 2 to 3 seconds compared to the Sierra. The reviewers thought that 2 second issue was enough to condemn the Novatel, which is bull$h!+.