I did some tests of my own. I tested with MA Caelum and 100 high seraphs. I did tests for growth 0, growth 1 and growth 2. I did 4 tests for each, that is 12 total and I am giving just mean value results. I only counted the number of still surviving seraphs on winter turns (10, 22, 34, 46, 58, 70), because they get diseased on late winter and by next winter all those diseased die. First late winter is turn 11.
Code:
turn growth 0 avg. growth 1 avg. growth 2 avg.
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10 100.00 100.00 100.00
22 87.75 88.25 90.75
34 78.50 80.75 81.25
46 69.00 71.25 73.25
58 62.00 65.25 67.25
70 54.50 58.25 61.00
It seems to me that the effect of reducing disease affliction is rather weak. I don't feel that taking growth just for the benefit of your old mages is worth it.
Also you cannot rely on growth to cut down on afflictions. I didn't calculate variances, but I'll say that the worst growth 2 result on turn 70 was just 50 mages and the best 69 mages; the worst growth 0 result on turn 70 was also 50 mages and the best was 61 mages.