After reading quite some feedback regarding the tile yield bonus, (some from here, some from reddit) I have made the final decision as to what polders in my mod will be.

With nine votes out of twelve favouring this option, Polders will now be restricted to three land tiles instead of two. This reduces the valid polder tiles by half. To counter this reduction, Polder yields will increase in some areas.

There are a few things that I was not willing to change. The food yield will not increase, and yields will remain equal for coastal and lake polders. That said, most favourable seemed to be the idea of adding a culture yield to Polders. This culture yield will however not be available until the industrial era. From now on, these yields will be available to polders:

At the Machinery tech, Polders will be unlocked giving a +2 food yield to coast and lake tiles. This +2 food stacks with the +1 food from the Lighthouse to coast tiles. The +2 gold yield remains at Economics and is unchanged from the base game. Polders get to their full potential at Steam Power, gaining an extra +2 culture. This can help either defensively or offensively on the cultural front in the second half of the game.

The change to Machinery tech and the second upgrade at Steam Power tech are due to realism: Polders have always been at the mercy of mechanical power to keep the water out. In the middle ages this was done with windmills, (Machinery) and later on it was done by steam engines. (Steam Power)

This will be the last change to the polder improvement for a while. Any future updates will be towards AI and technical issues. However, please don't stop posting feedback on the comment page of this mod. Also: should you really dislike any aspects of this mod: the XML files are clearly commented and with some creativity you should be able to tinker with them yourself. Please keep any edited version for private use.