I think its irrelevant what they were designed for as the banshee is based on 35yo ideas. most likely it wasn't designed to go much above 9k or so, atleast that's what the engineers probly had in mind at the time. that's not far off from what a modern stock 250 is designed for. it might be closer to the low 8000 but still not far off
the point to my previous statement was only to say that the piston speeds generated at 10k by a 68mm stroke are still below the mechanical limits of the crankshaft assembly, atleast for short to medium periods, assuming you have good parts. whether or not your particular engine will have the ability to run up that high or sustain those engine speeds for miles and miles is hard to say. if you have experience with other Yamaha twins like you say, then you should have no trouble sorting it all out