more than 25%. Remember that the BRT trains are almost 2x the size of the IRT trains, and consequently hold about 2x as many people.
An IRT train is 510 feet long, a BMT train is 600 feet long. The IRT trains are 8'9" wide, the BMT trains are an even 10 feet. The difference in area is about 33%. So the total capacity increase will be something like 33% instead of 25%. Not that huge a difference. The main problem is that they're building only two tracks on Second Avenue, and that the service on those two tracks will have to share the Broadway express for a long while.