I just installed the 5.5 beta (OS X). When I route signals through the H8000FW there is an increase in output levels. It is easily apparent; I'd say it is about +3db. All the input and output levels on the H8000FW are set to 0.0 dB.
My setup lets me switch my signal routing to the H8000FW, as necessary, so it is easy for me to A/B it. Nothing else in my setup changed, and I've carefully inspected the input/output levels on my other gear and they are all set to 0 dB. I'm sending signal in and out on the AES inputs from a Prism Orpheus. No Firewire cable connected.
I noticed this as well only I am using it as an asio firewire device with the firewire 8 track routing preset. I did as you instructed and loaded both DSPs with the thru preset and set to 100% wet. The output is close to 5.5 db hotter than the input. I checked this by switching input/output in the meter section and then lowering the output level in the output section until the output meter appeared to be the same as the input meter. I was recording 24bit 96k, keeping the meters in the green.