In this case the main question is whether people would usually say
rubberier or
more rubbery.
The answer is fairly clear. In the 14 million word iWeb corpus,
rubberier appears 11 times, but
more rubbery appears 233 times. Other corpora produce similar results. When
rubberier is used, it doesn't seem wrong. An example from a book called
Cincinnati Food: A History of Queen City Cuisine:
Many commercial hams are injected with brine to make them cheaper, which also makes them rubberier in texture.
Nevertheless, the word is rare, and will be classed as such in Chi from now on.
Rubberiest is already rare.