I would definitely do a stage 1 kit on a 100th anniversary sportster. Very cool bike!! Exhaust will look and sound better at the very least and with the air intake your definitely going to feel an improvement. In my opinion We never get back what we put into these bikes whether it’s a fat boy or a sportster, we know that going in. But it sure is fun making them our own!
You didn't say how much it would cost, so it's hard to say if it's worth it. I don't think you'll get that much improvement with a stage one for the 883, but if you really want to see a huge difference look into a 1200 kit upgrade. Now, for me that would be worth the expense.
Here's what you should ask yourself: How often do you ride with wide open throttle and wish you had more? If it's a lot then you should probably consider a 1200 upgrade or a different bike altogether.
On the other hand, even though you don't really get much, if any measurable performance increase with a stage I, lots of people "feel an increase in their butt dyno" and that makes them happy.
In my opinion I would only do a stage one on an 883 if I was going to change the exhaust to a 2-1. I think this is the only way that you would feel a difference, thus making it seem like it was worth the investment.