With my 3250HD (I believe it's the same with the Pace551HD), there is no padding at all (null packet). The stream is not constant. It's simply to big, from time to time, for the device to handle correctly and you missed some packets.
With the old OS (1 year ago), everything was fine. I missed almost nothing with the standard drivers for windows. Today, windows drivers missed too much to be usable (some SD are OK, but forget it for HD) and Linux (and Mac) drivers works better but even there, when recording high bitrate HD, you will missed a few data block. When the OS changed, they gave more priority to the system resulting in poor firewire output. When the channel has a standard bitrate (like TSN SD and RSN SD), it's working OK, even for windows (I recommend to test with one of those).
By the way, for the 8300HD, it's only working with recorded shows, not direct viewing (the video and audio streams are there but they miss a few system streams to work correctly, like PAT and PMT).
There is some 5C protected channel. Here is the list :
and a few others... There is nothing you can do for those.
The PACE551 may have different drivers than my 3250HD so it may works better (I don't have one to test). But be aware, this terminal needs a updated OS to work to with the SDV. So, in the next months, his firewire output capabilities may decrease.