Did a quick comparison on these three bikes in stock form as provided by
Feel Like A Pro track-bike rentals. Same tyres were used on all bikes. I selected same track, Thunderhill East (3-mile) and picked same rider in races in same month, 1-year apart.:
Ninja 250R = 2:30-2:33
Ninja 300 = 2:23-2:26
Ninja 400 = 2:13-2:18
Apparently 400 is significantly faster than 300 is over 250R. However, these times spanned 3-years, so I suspect lot of difference is also due to rider improving their skill as well.