Interestingly I first assumed that SDT (Shadow token) to SDC conversions reduced the available anon I/O in the system (since SDC > SDT increases it)
In actual fact converting Shadow tokens (SDT) back to SDC increases the anon IO yet further
could u explain how that works smooth?
It's false. Nether increases anonymity in the system, because they provide visible tracing points on the transparent chain. You are perhaps suggesting that SDT->SDC increases anonymity because there is another step of mixing and stealth, but this is mistaken, because the alternative is not nothing, it is spending those SDT by sending them to someone else. which gives the same mix+stealth, but minus the tracepoint.
Your best case for anonymity is really to just use SDT exclusively. Long chains of mixed transactions with stealth addresses at every hop = anonymous.
If you use SDC for the purpose of interoperating with BTC-based APIs, fair enough that may be a worthwhile tradeoff, but it doesn't improve anonymity at all, it decreases it.