Nathaniel, I have a 2000 25 Johnson with a primer system that I did this to, and it made a big difference. My original carb was part # 5000883, and I replaced it with a #0439377, 30 hp carb. I got these numbers off of evinrude.com, under the parts section. I cross refenced everything, intake manifold, primer, I even crossed the drive shaft numbers to make sure I wouldn't over load the gearcase. It bolted right up.
Before the swap, I was getting about 21 mph and 4900 rpms with a heavy, loaded 1648. Now, I am getting 25-26 mph, and just a hair under 5500 rpms (max for my motor). And, the OB was and still is set up about as optimum as possible.
It should be doable for you, just make sure to cross reference and be sure the rest of the OB can handle the extra ooomph!
Good luck!
Skidz