OK.... I use 80's/50's as that is what I own persl.--my persl. choice,,,,,its also nice for multiple dive days
80's with 50% is a good tank for multiple dives. (I've never used 50's, so of those I cannot speak.)
'Optimum' holds many different meaning to many folks....to me persl. it is optimal to use larger than 40s for deco.
When your deco/stage tanks are all slung where they are needed, it doesn't matter all that much. I find cat 40's hang like bricks, but they are usable. (and I imagine steel deco bottles would be around the same)
It becomes more of an issue if you have to swap tanks in/out.
For my example of the Athel Viking dives.
Diver removes leash with 50% and 100% bottles. (and holds that in one hand)
(This is neg by a couple of pounds, but the leash makes it easy to hold)
Diver then removes 80 stage and clips this to the leash.
(The 80 is pos and the 40's are neg, but the leash "system" is close to neutral, so if you drop it...it doesn't drop like a rock, taking your deco bottles with it.)
Diver then takes the 40's and clips them to his harness. (This is the dangerous part. This is where it is nice to have a support diver...to get back on topic
Now, If he drops the leash now...no biggie, those tanks are done.
Now, change all those luxfer tanks and make them steel bottles. It changes the dynamics of the whole swap.