Same rim size, and I use all the tire on my Ninja 300 too. Do you want a picture?
The Ninja 300 has a 4" rim. If you ask most of the makers of 150/60 performance tires what the best rim width is, the general answer is 4.00" - 4.25". The 150/60Rx17 Dunlop Alpha 13 was specifically designed for a 4" rim (as on the RC390). The design rim width is documented by Dunlop.
You know, the CBR250R and CBR300R use the same IRC Road Winner 140/70 tires that the ninja 300 uses. The difference? the CBR has a 3.5" rim which is more suitable for a 140/70. On the Ninja 300 with the 4" rim, it's a budget concession only, and people need to stop saying it's the best size because it's the OEM size. Those tires are on there because they are cheap.
Last weekend at Blackhawk we had a rider on a 300 with Bridgestone 140/70 S20's. Even with decent body position, he ran out of edge on on the 140/70's and almost lost the rear.
Hey by chance you're not Bruce Barry, are you?