For years growth rate issues have plagued spiggot. 

Where the modifiers seemingly did not effect the server if increased and required significant decrease to reduce. 

Today I have discovered why. 

Within "BlockCrops.randomTick",  there is an if statement as follows:

if (randomsource.nextInt((int)(100.0F/ (float)modifier * (25.0F / f)) +  1 ==0)

This is catastrophically wrong. 

The vanilla probability of crop growth is 1/(25 / f +1)

A modifier to this chance would be 100/Modifier[1/(25 / f +1)]{*}with 100 being normal growth.

This is NOT what occurs in the if statement above. 

(int)(100.0F/ (float)modifier * (25.0F / f)) is where the problem lies. 

This value will always come out as an integer rounded down. I hope it is clear to see that, say for a case of modifier = 100 and f = 10

that (int) 100.0F / (float)100 * (25.0F/10) < 1 and hence = 0. 

Hence, most cases (until modifier is equal to or less than 40) the growth chance will always be based on:

if (randomsource.nextInt(0 +  1 ==0)

Hence a 50% chance as a random int between 0 and 1 are chosen. If 0 , then true. 

This is a bug that, considering forum posts about the issue, has been persistent for years. 

As such I will offer my personal recommendation at a fix below: 

int chancevanilla = (int)(100/(floor(25.0F/f)+1))-1; //%chance in vanilla MC

int ModMult = (int)(modifier/100.0F);

int chancemod = (int)(chancevanilla*ModMult;

if((randomsource.nextInt(99) <= chancemod) // rolls a random integer between 0 and 99 (100 numbers).// If chancevanilla or lower is rolled, then true. 

I.e. if chance vanilla == 33 and then the modifier was 200, then chancemod would be 65 and hence have a 66% chance of success (66 choices lead to a true if statement out of the potential 100 ints between 0 and 99)