I think Stewart has a good point. Also, Saga Quests are not part of the bank. Therefore, the most economical way to hit them is with wraiths, which takes FOREVER and a day and then some in the 20s.
The most efficient BG levels are 70-90, though 50s and 60s are still pretty good. Above BG 100, the efficiency drops. While the minimum bank level in resources increases by 7% per level, I believe the maximum payout increases at less than 7% per level, and by the time you're in the 150s, and certainly at 190 (or is it 200 now?), the maximum payout is less than the minimum bank needed to trigger a large payout, which means you will never empty the bank in the higher BGs. The law of diminishing returns at those high levels really doesn't make it worth it, IMO.
To receive as many efficient BGs as possible, you should ideally stop increasing your BG levels after 100 or so. Since the only way to access new Saga Quests is to increase your BG levels by 4 over the previous SQ, going above BG100 to access any added SQs will get increasingly inefficient, and result in fewer efficient BG levels.
Without food restrictions, at those high levels, its really far better to just do
As far as the legendary units, you are still likely to get more power in troops alone by sticking with
Still, if new SQs were added, there would be no requirement to do them, so by all means, add them. Maybe I'm missing something, but I only see a disadvantage in doing them. To each their own.