Great discussion and details on the Stormscope.
I'm SHOCKED that anyone is still flying with XM weather, thinking that it is current as of the time shown. That time is just when it was released, and does not take into account the time required to gather, digest and create the composite picture, thus the 6-10 minute average delay that is longer during active weather.
Coming home through NC yesterday in the early afternoon, Center advised me of "heavy to extreme precip" ahead of me, just after I asked to deviate left around the buildup. It was nice and tall and not too dark, with a nicely developed anvil on top. Stormscope was clear. I deviated left around it and a few other tall CB behind it, and reported back when I could see from the side "no visible precip beneath the buildup." Nary a bump to be felt except when clipping tops.
See http://flightaware.com/live/flight/N444DJ and the course shift is very visible. I think the 430 showed about 15 left of course at the max as I paralleled along. I'm not sure what weather flightaware presents; the anvil was spreading off to the east, so I took the longer way around and went left, up to almost 40ยบ at one point. I took a picture with it in the distance, but don't have my camera with me right now.
No XM, so I can't comment on what it showed other than Center's "heavy to extreme precipitation" comment in response to my deviation request. So I asked again and deviation to the left was approved as I was turning the yoke.
Bases were mostly 5-6000', and tops rose from below me to 9-10,000 the further NW I traveled; this one was much, much taller. I probably could have climbed to 10K for better distance visibility, but with the temps aloft she wasn't climbing too well reaching 8000 and I didn't have much trouble spotting buildups. Think I logged 0.3 actual, usually in bursts of a few seconds to 3 minutes.