A.    Symbolism of the 215 years of the patriarchs vs. the 430 years of bondage.  This is problematic, if there is any symbolism at all.  I am skeptical that there is, but since people do this then let us try to discern it.  If there is then it must hinge on two things.
1.    The concept of curse/retribution vs. the concept of blessing.
2.    The concept of something being double when compared to something else.
These two concepts actually come together in quite a few of the legal passages of the Old Testament.
1.    Concepts Laws Concerning Theft: Gen. 43:12, 15; Exod. 22:4, 7, 9;
2.     Laws of Slavery: Deut. 15:18
Based on these two types of passages then I would conclude that if symbolism exists it would have to somehow be linked to these types of concepts.  Was it that they were being punished for some great wrong(s) such as the sale of Joseph their brother or the immorality that the people were commiting like the story of Judah and Tamar? Or was it to show them the nature of slavery that it is twice as hard as being a free man.

B.    Symbolism of the Patriarchal Ages
Nahum Sarna
5 x 5 x 7 = 175
6 x 6 x 5 = 180
7 x 7 x 3 = 147

 To me this is a curiosity but nothing more.