It has long been established that spring training stats are not particularly meaningful, reliable or predictive of future performance.  Will Carroll at Baseball Prospectus wrote a piece about this a week or so ago and there have been other discussions of it, as there are every spring.  However, there might be one limited exception.  John Dewan, owner of Baseball Info Solutions, most famous for his work in fielding statistics has done some research which shows some correlation between high spring training slugging percentages and improved performance in the next season.  

In his 2006 Baseball Forecaster, he wrote, "A hitter with a positive difference between his spring training slugging percentage and his lifetime slugging percentage of .200 or more correlates to a better than normal season."  He recently expanded on this a bit at actasports.com:

A few years ago we discovered that there is a way to use spring training stats to predict future performance. We took all spring training hitters and found that, as expected, about half of them do better than their career norms in the upcoming season, and about half of them do worse than their career norms. However, when we chose only those players doing exceptionally well in spring training, we found that about three-fourths of them performed better than their career average during the upcoming season. Our definition of "exceptionally well" was slugging 100 points higher in spring training than their previous career slugging percentage.

This is still relatively new research and there is some debate about how meaningful this is, but pretty much everyone agrees that there is something here.  Here is what Joe Sheehan at BP had to say about it in a recent chat sesson:

Grasspike (NC): You say Spring Training performance means very little, and for the most part, I agree. However, isn't there a correlation between breakouts and a big spike in slugging percentage?

Joe Sheehan: Yup. I shorthand it, but the one thing--I think this was John Dewan's work--is that a 200-point jump in slugging over career marks is supposed to be real. It's been proven, although I still the variable comp is a major distorting factor.

So, there appears to be something to this, but before we get too excited, we have to remember that Dewan isn't really predicting a "breakout."  He is basically saying that if a player has a slugging percentage much higher than his career SLG, then he has an increased chance that he'll have a better season than his career average.  Basically, he's saying the player will probably get better.  That doesn't tell us a lot, but it's something.

One thing that you have to be careful of is very small sample sizes in career numbers (for a guy like Callaspo with fewer than 200 career MLB at bats or for a guy like Olivo with only 30 spring training at bats)  But, with caveats set and grains of salt ready to be taken, here are some Royals for whom this might be relevant:

Player Career Spring Difference
Jason Smith 382 711 329 (Little Poppy showing some pop off the bench!)
Alberto Callaspo 280 532 252
Miguel Olivo 405 633 228
Billy Butler 447 667 220
Ryan Shealy 402 612 210
Damon Hollins 417 540 123
Mark Teahen 429 547 118