Ortiz has played in 82 postseason games, about a half a season's worth. He has had 357 PAs and 295 ABs. Still not large enough for the numbers to be completely stabilized, but a fair-sized sample.
His numbers are as follows:
Career - .285/.379/.547/.926
Postseason - .295/.409/.553/.962
There's really not that significant a difference, with the largest difference coming in OBP. That difference in OBP would work out to be about 1 extra time on base each week. Most of those extra times on base come from an increase in being intentionally walked in the postseason.
In the postseason, Ortiz has been intentionally walked once every 32.5 PAs. For his career, he has been intentionally walked once every 49.7 PAs. Being intentionally walked makes up a little more than half the difference between postseason and career OBP.
Also, in the postseason, he has hit a HR once every 17.4 at bats. For his career, he hits a HR once every 16.3 at bats. His HR rate is lower in the postseason.
There is also a stat at Fangraphs called, oddly enough, "Clutch". For the postseason, his Clutch rating is 1.12 and for his career, it is 1.13. Both are very good. However, if you look at the individual seasons, he had more seasons below average in terms of clutch than he had above average. In other words, back to the original point, clutch is not a repeatable skill.