I did this exact change to my 99 road king. Its called a "compensator gear" and I still have it.
This gear change will make it run at a higher rpm in top gear,,unless you go with a six speed.. I called Baker and and talked to one of their employees that had set his RK up the same way.
The Baker rep actually set up a custom gear set so that the ratios preformed best in each gear based on the Dyno sheets and even told me which tooth to purchase on the compensator gear . At this same time I had Zippers send me their 107 kit. It came in at (if I remember correctly) about 120 hp and about the same in TQ its on their web site.
The only down side that I experienced was that it seemed to make a greater vibration in all gears. The up side is I was very rawly beaten at the red light races and you will even get to smoke the V-rods.
The other change you will need to make is to find a very sticky rear tire and you are going want to change the spacer on the rear wheel if possible to get a wider tire back there. When thinking of engine mods with this tranny set up,,, because its geared lower your going to want a motor that likes higher rpms to take full advantage of that gearing. For instance my red line around 6k and in the first, second and third gear,, these shifts would only a second or two long so you have to practice it to stay out of the rev limiter.
I just smoked the rear tire until I got a wider tire and softer compound, once that was finished, I could wheelie deep into first gear with a great hole shot, and the six speed still gave me a great top end. OH YEAH one more thing..... expect to spend a lot on rear tires. You will only get a few thousands miles got of them.
Hope this helps