I can't believe you'd see faster growth with Acros as a high KH. You say SPS so if you're not growing acros you can ignore this.
At a high KH you are playing with fire. If any nutrient runs too low the tips will burn (die at the ends). I see incredibly fast growth at 7KH with my acros and they can tolerate more parameter swings at a more natural level. If you run a delightfully filthy tank that's older and you don't obsess with low nutrients, PO4 especially, it may work just fine, but you're playing with fire IMO. :)
Acros only though, other SPS seem to not care a bit.
Interesting... I don’t have any acros but based on what I’ve read I will keep my KH on the lower end of the scale anyways. I don’t want to risk it even for a biscuit