Shocking amount of 'short-circuiting' going out that exhaust port. If you can get that fixed, I'd expect a bit of a bump in power and fuel economy.
That pattern on the piston of the primary transfers goes to show how important the angles are in the walls of the transfer ports. Most guys just worry about 'numbers' when a change in the angles could net them even more.
If you can reform the tunnel and the angle closest to the exhaust port to turn it more towards the back wall, it may help correct the short-circuiting. Even if the tunnel becomes a bit larger than you want, the exit of the port (especially when it first cracks open) could fix the problem.
Years ago, and it was included in Blair's work, there was some porting done to a series of Yamaha DT 25o jugs. They kept the same time/area of all the ports (exhaust, transfer and intake), but changed the port configurations and angles. The difference between the best and worse performance was a cool 15% difference in power. That difference was across virtually the entire powerband.
I look forward to a picture of the pattern on the piston after you finish porting.