Put some miles on the x99s, here's what I think:
NOTE: This is coming directly of 101a Ricta Natural Slims (52mm Asta - hardest wheels I own and a terribly underrated wheel; they do wear much faster than anything else).
Out of the wrap the x99 are much whiter, they have an almost milky look them.
They still have that 'dragons/x97' feel, slightly bouncy and quiet (but less so in either) - they're noticeably harder, like going from a 99 to a 101, you can feel it when just rolling and for sure when sliding; they're louder compared to the other two and actually have some bark to them when speed checking or sliding.
I rode them in the same spots I rode Dragons and X97s (and spit 101/99 - Ricta 101)
They are ever so slightly less smooth than the 97a (and for sure Dragons)? But not enough to give a fuck on rough terrain or really notice tbh, it's so difficult to tell how much rebound you lose as they still roll over rough terrain with ease compared to a regs 99a or the x97; they actually have/produce 'chatter' on really chunky shit, where the 93/97 would be much more muted/softer. On smooth terrain they are just as smooth/quiet as the x97s but chirp more...less 'grrrrnnk' (93a/x97) and more 'scrriiiinch' (wheel sounds

).
Much less gummy on pinch grinds/5050, bluntslides were much, much more predictable (very very close to spit 99a fee)l - love'em or hate'em the 103a stf are bluntslide kings - faster overall than the 93/97 - AND, if you skated either of the other wheels you know that squeak when you grind? When you sort of 'bind the grind' Haven't heard it yet.
Park setting, I'd still opt for an SPF but that's me, I'd never touch a 99a in a park unless it's shit. Those of you that skate spit 99a in a smooth park setting these are still not the wheel for you but they are faster and overall better on smooth crete than the 93a /97a formulas.
x99 top / x97a bottom 9skated these 54s only a handful of times so they're not that yellow from age

I'd ride these over any 99a in the streets and would just say fuck it and ride them in a smooth park (just not for transition) just not to have to deal with swapping wheels.
Not sure why anyone would ever ride the current 99a STFs again; wouldn't be surprised if they toss out a 95a Dragon to sort of balance the options between the branding.