I crossed Buff and White years ago.It took about 4 years of backcrossing to pure Buff males to get the color back and by then the better type was small,not any better than strict selection within the pure Buffs. There was a high percentage of culls with this project. If you can find,great typed Black Leghorns, Black tailed Red Leghorns ,or Brown Leghorns.(but use outstanding show type Leghorns),.you could end up with some richer buffs using repeated backcrosses to buff males.
It is possible that if the F1 from a Buff x White cross were bred together and really huge numbers bred to get the f2,perhaps a tiny percentage (1-5 %) would have type and color if you were lucky.White in tails ,wings ,and whitish under color will be persistent and slow to yield to solid buff tails.You may see blue,or faint barring feathers and green legs in the f1.
Any time you cross with buff,be prepared to have lots of culls and allow for years of work ,if you expect to benefit from the process.. I sometimes think just looking for individual pure Buffs that have some traits you need,will do you more good.
In relation to the concave Leghorn back.It is the saddle and tail coverts that fill in the section just before the tail.The more feathers in this section (multiple feathering gene),the better the chance there will not be an aburpt break just before the tail making an angle instead of a curve..Dan