There are a couple of things that will cause this to vary.
Sail material, construction technique and how the panels are laid out.
Sail material varies greatly and will directly effect the way the kite performs. If you use the same type of material as the original then this shouldn't make any difference. But if you replaced an Icarex material with standard Ripstop or even mylar then that will greatly effect not only the way the kite flies but also the wind conditions that the kite needs to fly properly. Each material will have a different weight which is very important to how the kite performs in different wind conditions.
Construction Technique: How the kite is made will also make a difference on how the kite flies. Every seam in the kite adds weight, if you are planning on doing an applique or adding in additional panels, this could increase the weight of the kite. Also, how those seams are put together will make a difference on durability and sail stretch. Reinforcement material is strategically placed on the kite to help maintain the shape and durability of the sail. Changing any of these could be an issue and will surely effect how the kite performs.
How the panels are laid out: High end kites will usually incorporate material biasing. This is a technique that aligns or cross aligns the bias of the material to give a desired effect of the kite while flying. Biasing the material can improve strength and reduce stretch. Certain panels are sewn in certain directions to give the sail strength and reduce the need for additional reinforcements. This too will directly effect how the kite performs.
Personally I am not a kite builder but have learned these things from talking with many professional kite builders and designers. Perhaps one of them will be able to give more direct and technical input on this.
I don't want to say that you shouldn't build a sail, in some cases you can rebuild your sail and actually improve on the performance. If you did decide to change from a nylon to an Icarex sail, you might be able to improve the light wind capability or you could change the distribution of the weight of the kite to improve certain tricks.... By building your own sail, you should be able to improve the looks of the kite - at least in your own eyes anyways.
Hope this helps.