Little did I know, but Apologetics Press has done much the leg work concerning the time of the slavery of the children of Israel in Egypt. Thanks to Joe for the heads up on that and the link. Here is their explanation, but it reconciles with my thoughts.
The ESV Study Bible notes in a similar way:
came 430 years afterward. Paul is apparently referring to the Septuagint translation of Ex. 12:40, “The dwelling of the children of Israel . . . in Egypt and in Canaan was 430 years,” which would mean 430 years from Abraham to the exodus (the Hb. text does not include “and in Canaan”). Another explanation is that Paul is not counting the time from the first statement of the promise to Abraham but from the last affirmation of that promise to Jacob before he went to Egypt in Gen. 46:3—4. This method would then count the entire time in Egypt as the time from the “promise” to the “law.” If this is so, then Paul is relying on the Hebrew text of Ex. 12:40 to affirm a 430-year stay in Egypt.
I think taking Paul in a straightforward statement that from the promise of Abraham to the exodus was 430 years. Then we simply need to understand Exodus 12:40 to be saying that the time of the sojourn (from Abraham to the exodus) was 430 years. Thus, the Egyptian slavery was only 215 years, not 430 years.