Hi!
The 300/300 diamond stone are awesome because the shape never warps, which means you'll never have to fix it with a stone fixer. The 300/300 stones will make fixing any minor chips easy. After very long periods of use the 300 side will smooth out a little and parts of the stone may become about a 500 grit.
Oh no, sorry I meant that you can ruin the blade if you change angles too much while sharpening. Granted that stand true for any sharpening stone, but especially for something as abrasive as these diamond stones.
I don't think I quite understand your question. Will it increase the lifespan of the knife? No, actually it might make it shorter if you use it improperly and shave too much of the steel off while sharpening.