A premix ratio of 32:1 is standard for 250cc class motocross machines like your Yamaha YZ250. You can go as lean as 40:1. I run 32:1. I use Honda Synthetic HP2 premix because of the power valve. Yamalube is great stuff too.
As to whether it will take 10 minutes to redo the top end thats debateable.
First on your YZ250 you have liquid cooling which will add to the total time to get it back on the air. Plus you'll have to remove plastic such as the rad shrouds and possibly the fuel tank, etc. There is also the power valve to consider and adjust when a cylinder is removed not to mention the reed valve. Plus mid nineties and up motocross machines have a nickasil plating in the cylinder bore stock. Unless you are in a major metropolitan area that has a specialty shop that specialize in this, normally you have to send the cylinder out to have it re-plated or bored like to companies like US Chrome, Pro Circuit, Kustom Kraft, etc. So there will be waiting time for parts to consider.
Another thing you can do with a 2 stroke just like a 4 stroke is run a compression test if you suspect cylinder, piston or ring wear.