radiant floor retrofit vs. new install

I've been thinking about retrofitting a hydronic radiant floor heating system into my existing house with hardwood floors everywhere except the baths. My contractor is experienced with the staple-up systems.
Compared to "infloorboard" (http://www.warmzone.com/hydronic-radiant-floor-heat.asp ), does anyone have a feel for the efficiency lost by being under the subfloor vs above it? Or is that part about the same ... given that you'd lay another subfloor on top of the thermal board before putting down hardwood planks. (yes? or only for tile?) What about over the tiled rooms?
The other issue is that being on top of the subfloor gives better insulation from below, right?
I know the retrofit cost is going to be much more than an original installation cost, but what I'm trying to determine is (ROUGHLY!) how much more expensive will operating costs be? Slightly more than a new install, or MUCH more?
thanks -frank
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That product looks just like WarmBoard. www.warmboard.com

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