+1 also - i suggest looking into the reverse. if you are bullish, run your simulation on writing OTM puts, and buying further OTM puts. that has a much greater stability.
Another potential problem could be the unlikely event that the short calls spike in value higher than the long calls for a brief time. Hard to believe but something to be aware of. http://www.elitetrader.com/vb/showthread.php?s=&threadid=225150
Here's anther way to look at all of this. I buy a long call. You buy the same long call but you also sell the next OTM strike against it (a vertical spread). The difference b/t our P&L will be the short call's performance. We all know that a naked call is profitable below the total of the strike plus pemium received. Above that, it loses, possibly severely. The further the UL goes up, the more it loses. Fortunately, you have a long call so for the most part, your profit is b/t the strikes. Above the short strike, they offset. I don't have that problem. As long the UL rises, I do well. The UL's downside is a different story but not relevant here. Now you can conjure up some upside scenarios where the spread outperforms (certain price/time/IV levels) but they're going to be few and far between. To the upside, the bang for the buck is with long, unencumbered calls, not spreads. OK, that horse is dead!