But taking the model of "honest" staking at face value, at 49% you can only hope to maintain control for a limited number of blocks. At 50%+, you control the chain forever. That's clearly more than 4% increase.
Why is that the case? Even if it were the case, it might just be a terminating condition and not imply that the entire relationship was superlinear.
e.g. according to this tool for calculating the amount of blocks you forge over 332 days given your stake:
https://www.mynxt.info/forging_calculator.phpThe relationship is linear:
stake,forged blocks
1000,1
10000,13
100000,132
1000000,1327
10000000,13273
100000000,132732
1000000000,1327326
Over a period of 332 days, there are 478080 total blocks generated, so you would win 100% of the generate blocks somewhere between a stake of 10% and 100% of total supply. Interpolating, that puts full control at 360182803 NXT
which is 36% of supplySo, this implies that NXT is actually vulnerable to a 36% attack.