Would you say given the relative size of the Market Caps and Volumes of STEEM and SBD that this would have just a marginal effect if any?
It is hard to say. The most obvious answer is that SBD has limited effect on STEEM given the relative sizes of the markets, and that is probably true to a large extent. But not necessarily entirely.
First, the market caps don't directly imply how much supply is actually available in the liquid price-setting markets. For example, a huge portion of STEEM's reported market cap is locked up as SP. Furthermore when SBD gets overvalued this serves to narrow the difference (a natural stabilizer). In addition, reported trading volumes on crypto exchanges are very suspect. Finally, the argument is somewhat circular. There is demonstrated huge demand for stable value within the cryptocurrency ecosystem (Tether, also rapid uptake of other solutions such as DAI) which dwarfs STEEM's market cap by about 10x and additional demonstrated huge demand for stable value outside the cryptocurrency ecosystem. Drawing some of that value (even a tiny fraction) into SBD could result in an upward spiral that lifts the market cap of both SBD and STEEM (potentially by a lot).
I see SBD as a 'product' of the Steem blockchain (along with others, such as value transfer, censorship resistant publishing, rewarding contributions, etc.). It is a product that has a vast addressable market, possibly dwarfing the others.
I have been thinking about the burnpost initiative from another angle and I always saw [burnpost] as reducing the supply of SBD
How would it do that when it immediately dumps its SBD on the liquid market? Secondarily, powering up (which @burnpost does) increases the STEEM market cap which further increases the supply of SBD (if only by a little). So overall I don't understand this idea at all.
Would the fact that it is reducing the debt on STEEM
I don't understand this question, including what you are referring to here with the word "it"
I'm also skeptical of the word 'debt' by the way, especially when used without further clarification. SBD has some properties that are analogous to debt but it isn't a perfect analogy by any means.
Thanks for those clarifications on burnpost. It makes more sense to me now.