The ninja 400 in track form is within 10 pounds of the 300s. Class limit for CSBK is 320 for the 300s, 330 for the 400.The regional organizations don't have a way of enforcing either a horsepower limit or a weight limit, and I fully realize that being up against a Ninja 400 is going to be an uphill battle. (I got beaten on my FZR400 by a teenager on an almost-stock Ninja 400 at the last SOAR round.)
This class (nowadays) is meant to apply to motorcycles which, in street form, are eligible for the european A2 license class - max 35 kW/47 hp, max 0.2 kW/kg. It's unclear if that horsepower is to be at the engine or at the rear wheel. Engine horsepower is essentially unmeasurable, so anything less than 47 rear wheel hp from the factory is fair game as long as it also meets the power to weight restriction ... I believe the R3 is hitting the power to weight ratio; the Ninja 400 is probably heavier so it can have a higher power. Same issue with engine-power-vs-rearwheel-power applies to the power to weight ratio.
At WSBK level, where they have a comparable class, they have been applying all sorts of restrictions to try to even out the class. CSBK requires the Ninja 400 to use an intake air restrictor.
I didn't go the Ninja 400 route because I would have had to buy a new one $ $ $, and the aftermarket parts support doesn't appear to be quite there yet. All sorts of stuff is available for the R3.
I'm beyond chasing championships, too olde for that. I just want something that's fun to ride, and works as it should, and is somewhere near competitive.
Brian. I'm 55 and currently chasing two championships at RACE on the R3. So age isn't a factor and the R3 is a huge amount of fun. Those pesky 400s have shown up during the season and made podiums a lot tougher though.
Last round at Shannonville is in two weeks. You should come out and watch the little bike fun if you can't race it.