I think it is a matter of what your asking the bike to do. As Noglider himself notes changing the style of the handlebars will drastically change the character of the bike and how you intend to use it.

If yure simply looking to get more performance out of a bike I was always taught that changing the wheels and tires gave the best upgrade for the money.

For things like changing the size of the bars and stem or saddle have more to do with making the bike more comfortable than performance.