I've used the standard Hennessy Super Shelter, incl the underpad, for about 300 nights of hanging here in the north country (MI).
I was also thinking of getting the newer silvered bubble pad to use in conjunction with my existing setup -- for better insulation. Note that I use an Explorer Deluxe Asym Classic.
When I called up Hennessy to ask them whether this is a good idea (I think it was last year some time -- 2012), the guy on the phone I spoke with recommended against using the two systems at the same time. He kept saying that they are two different systems -- each is good in and of itself -- but their official recommendation was against using them both at the same time. This didn't make sense to me, but I took their advice, just the same.
I've upgraded to an Arrowhead Potomac UQ. I've only used it two nights so far. It seems to work well. My strategy is to mostly use the UQ (i.e., not the underpad). For maximum warmth on the coldest nights, I intend to use the UQ in addition to the underpad. I haven't yet tried using the space blanket in addition to the UQ, but that should improve warmth as well.