Obviously, the most effective pitcher turned position player was Babe Ruth. In 6 seasons as a pitcher (includes 5 games over 4 separate seasons he pitched for the Yankees), he was 94-46 with a 2.28 ERA in 163 games (147 starts). And the fact that he became the best offensive player ever (after he went to the outfield) makes a comparison to him unfair.
The easiest story to compare what Ankiel and Loewen have gone through is that of Johnny Cooney, who played 20 seasons in the majors from 1921 to 1944. As a decent, not spectacular, pitcher for the Boston Braves, his skills began to decline when he reached 29-30 years old. Cooney was known to be a very good hitting pitcher and played his share of games in the outfield when he wasn't pitching. Finally, the Braves saw enough of him pitching and though Cooney insisted he could still hit, they released him in 1930. It still took him five years to become an semi-everyday player when he broke in with the Brooklyn Dodgers in 1935. He would spend the next ten years in the big leagues, retiring when he was 43. As a hitter for the Boston Braves (Bees), Dodgers and Yankees, he batted .286 with a .671 OPS in 1172 games and 3372 career ABs. He had 965 hits, 130 2B, 26 3B, 2 HR and 219 RBI. His most impressive feat was the fact that, in spite of being a former pitcher, he was impossible to strike out. He struck out just 107 times in his entire career, while not walking much either (just 208 times).
Very often now, we hear about position players becoming pitchers. This is usually done in the minors where MLB players such as Dave Stewart, Trevor Hoffman, Edwin Jackson and Kenley Jansen have made the transition. Scouts see the arm a young player has and they become a pitcher, but it is usually done at the lowest levels of the minor leagues. But, for every story that works out, there are many that don't; as those are the players who are never mentioned. One time Mets catcher Joe Hietpas, who appeared in one game in the 2004 season, attempted to make a comeback as a pitcher. He pitched for A Port St. Lucie in 2007 and AA Binghamton in 2008 before hanging it up for good. The odds that a pitcher could become a good hitter or a hitter can become a good pitcher are not very good. The odds are worse once a player makes the majors and switches. Adam Loewen has his word cut out for him, but he has succeeded in AAA, which leaves him only one more step.