Cowboys rushing total with Paul Alexander (7 games) - 194 carries for 958 yards (4.94 ypa)
Cowboys rushing total with Marc Colombo (9 games) - 245 carries for 1005 yards (4.10 ypa)
Dak Prescott with Paul Alexander (7 games) - 128 completions out of 206 attempts for 1417 yards (6.88 ypa), 8 TDs, 4 INTs, 23 sacks, 11.1% sack percentage
Dak Prescott with Marc Colombo (9 games) - 228 completions out of 320 attempts for 2121 yards (6.62 ypa), 14 TDs, 4 INTs, 33 sacks, 10.3% sack percentage
While the Cowboys won more after the firing, you can look at the stats and conclude that the offensive line play didn't really improve. Rushing efficiency went down, and the passing YPA went down even with adding Amari Cooper to the offense.
This is not a defense of Paul Alexander's performance as offensive line coach. It's perspective on whether the play of the group changed after the move was made. Leave the won/loss record out of it - that's correlation without causation once you look at the numbers.
If anything, the Cowboys should be in the market for a new line coach.