I think you have to be very careful when it comes to increasing the yield on a craft as it's probably set that way for balance reasons. If you think about it, when it comes to quickly producing crafted materials, it's the material sellers that really benefit and are the ones that are probably clamoring for it the most. It isn't simply a matter of convenience anymore but rather a question of how much product that they can produce per unit time. The motive here would be an increase in sales due to greater availability of their product.

I've completed and sold thousands of 3* and 4* end game crafts in ARR and HW and I know for sure that the quick synth speed ultimately isn't what bogs down those crafts. It's the time it takes to NQ the crafted materials that you can't quick synth. While slow, the quick synth speed is acceptable because you aren't crafting stacks of 99 materials (the ones that you can quick synth) at a time. Nearly half of your time is spent NQing crafted 3* or 4* materials and most of the remaining half is spent HQing the final product. I'd have a field day if those 3* and 4* crafted materials could be produced as quickly as a quick synthed material.

For things like achievements, it's a one time exercise anyways and you can probably sell the crafted goods too. I don't think it would be worthwhile to break the balance over that.