All this discussion about whether he can sustain his away numbers is confusing and frankly not as important as seeing how he performs in Busch stadium. After all 50% of his at bats will be in this stadium -- whether or not Great American park is better than Coors is not going to have as big of an impact as the switching of home parks.
Also the discussion of whether a park is a hitter's park or a pitcher's park I think is overblown when it comes to each individual player. Just because in general, more runs are scored at Wrigley Field than say Shea Stadium doesn't mean Khalil Greene will have a higher batting average in Wrigley than in Shea. Each individual batter is unique and will see the ball differently in each park. Whether it's because of depth perception issues or perhaps a more subconscious reason, batters will have different comfort levels at each park. Of course the size of the foul ball territory, the distance to the walls, wind directions, etc... will have an impact, but I'm not sure you can generalize park impacts on individual hitters.
Greene's career average at Petco is a woeful .228 over 1147 plate appearances. Unfortunately, his average in Busch is only .174, but the sample size is very small with only 23 official plate appearances. There is not enough data to make a determination either way, but the little data we have doesn't point to him being a .280 hitter.