More existing SBD means each of them backed by less STEEM. Even if the SBD isn't being paid out they still affect market price.

That is a good point, although the magnitude is unclear. I would say that SBD which is paid out and circulates has a much greater impact, but I can't quantify this or prove it. In any case, the numbers are currently very small. See below.

Proposal to directly burn SBD from @steem.dao is great way to reduce SBD supply but if it's funded it will interfere with other funded proposal (if my understanding on how the DAO work is correct).

Only to the extent that:

1. It is voted higher than other approved proposals, and
2. The total approved proposals plus the burn rate is higher than the maximum payout rate.

Currently the maximum DAO payout rate is about 1700 SBD per day and active payouts is only 230 SBD per day. So clearly there is ample opportunity to reduce the size of the DAO fund with burn without affecting other payouts, if desired.

Currently the DAO fund is about 170K SBD out of 7.3M SBD total, so the DAO is really negligible when it comes to the backing ratio. In the future that might not be true, in which case I would expect increased interest in some DAO fund burns, or funding things like @sbdpotato from the DAO.