I think initial values and single increments are the in the same range (e.g. a green mod can start with up to +45 offense, and any new level can add +45 offense, either as a new stat, or added to an existing offense stat).
If that is true, the max value of any stat is 5 times its max initial value (if all 4 upgrades add up to the same stat).
Single max values I've recorded are (together with theoretical max assumed to be 5 times that, and the best value I've personally observed, which is far from it since I've never had the same stat upgraded all 4 times):
speed: 5 (very rarely 6) - max 25 (30?) - best: 15
offense 45 - max 225 - best: 147
offense 0.52% - max 2.6% - best: ?
defense 9 - max 45 - best: 14
defense 1.56% - max 7.8% - best: 3.1%
health 409 - max ~2000 - best: 1076
health 1.12% - max 5.6% - best: 1.9%
protection 820 - max ~4000 - best: 1800
protection 2.18% - max ~10% - best: 5%
crit chance 2.23% - max ~10% - best: 3.5%
potency 2.19% - max ~10% - best: 6.5%
tenacity 2.08% - max ~10% - best: ?
Note that I didn't try very hard to maximize some values, as I think they're not worth it (e.g. tenacity). In my mind, secondary stats start to be noteworthy at:
speed: ≥5
offense +80
health: +500 / +2.5%
protection: +1000 / +5%
potency: +4%
crit chance: +4%
... and the other stats, never (defense / ~%, offense%, tenacity)
