Charlie Cook at National Journal looks at the decline of swing seats, a potential Republican advantage and finds redistricting to be a secondary factor.  So one conclusion can be drawn from the data:  It may have helped pad the majority but the Republican majority is not a result of gerrymandering.

The trend lines are clear. In 1998, we found 164 swing
seats—districts within 5 points of the national partisan average, with
scores between R+5 and D+5 (a score of R+5 means the district’s vote for
the Republican presidential nominees was 5 percentage points above the
national average). The data 15 years ago showed just 148 solidly
Republican districts and 123 solidly Democratic seats. Today, only 90
swing seats remain—a 45 percent decline—while the number of solidly
Republican districts has risen to 186 and the count of solidly
Democratic districts is up to 159.

…How did we get here as a country? Debates
rage at political-science conferences: Are voters aligning in
like-minded areas, or is blatant partisan gerrymandering to blame? Our
newest index points mostly to the former, which has in turn amplified
the power of the latter. In 2011 and 2012, redistricting diminished the
number of swing seats from 103 to 99. But when we factored in the 2012
election results, the count fell more sharply, from 99 to 90.

In
fact, setting aside redistricting, we found that 76 percent of
Democratic-held House seats had grown even more Democratic in the past
four years and 60 percent of GOP-held seats had grown even more
Republican. Some districts swung dramatically, mostly along racial
lines. Republican Rep. Hal Rogers’s 96 percent white district in eastern
Kentucky jumped from R+16 to R+25, and Democratic Rep. Loretta
Sanchez’s 82 percent nonwhite district in Orange County, Calif., moved
from D+3 to D+9.