Whether it's singles or doubles means absolutely nothing in this situation. ZSS is the same either way and if anything, it makes his TPhantom improvement more meaningful since the technique has to cover a greater area in doubles.
I'd call these good enough but your mileage may vary I guess.
This is a breakdown.
Whilst Konomi specifically stated something Kirihara cannot do, did he specifically state what Niou cannot do?
On that. It's most likely that Irie+Atobe is weaker than Atobe+Atobe. Syncho might just boost the ability much more than another higher tier characters. That'll be my guess.
And on Niou power, it's most likely that Niou power is low before U-17 and since he go though his mountain training, his stat boost so he can now copy some power characters, like Kabaji.
Fuji's washed-up. Don't expect to see him anytime soon. Konomi's doing a good job giving us shit from guys like Marui and Kite. No need to see anything from Fuji Stu.
Niou can blow rackets away as Kabaji but not as Tezuka. It's pretty obvious that Kabaji has higher physical power than Tezuka (that's why the mirror match was bad for Tezuka after Kabaji copied Hyakuren). I don't know if he gets 100% of the stat of the guy he copied but his stat is clearly tied to whoever he's copying.
While Atobe X 2 is probably the strongest combo since synchro seems to pretty much trump anything in doubles, Niou would be better served starting as Irie. He could also turn into Oni and just start doing BJKs. If he can blow people's racket away as Kabaji, surely Oni's BJK would easily do the same thing, and ni presumably has higher base stats than Kabaji.
Picking Irie seems like the safest because he has incredible tech, speed, and mental. He also plays in a style that conserves energy somehow (his endurance is 2, but outlasted Atobe). Atobe pointed out that getting into an endurance game against HSer is bad, and Irie clearly plays in a style where he can play endurance games despite his staggering low endurance rating. If you copied Oni or Tokugawa it is possible that you get a case where moves used by these two more powerful characters (based on reserve team ranking) uses up way too much energy for a MSer level endurance. But, there's no such problem with Irie. His endurance of 2 is likely lower than almost all MSer, so you know whatever move he uses is very safe to copy.
If Niou just did Irie in set 1 and Atobe in set 2, I think he'd have won the game a lot easier, and most importantly he wouldn't be at risk for sustaining career ending injuries. Irie's 'troll mode' is probably the most useful non main character support ability. Literally every move can be stopped by that when he just says 'just kidding!'.
Illusion can copy someone as long as Niou's maximum stat exceed them, and it is limit by both their stat and Niou stat.
Niou can copy Tezuka in National, but since his tech aren't high enough back then, he can't do ZSS yet.
Niou copy Tezuka, but due to some Illusion limitation, he is unable to use his full strength at will and can only use as much strength as Tezuka have (3.5).
When he's copying Kabaji, he's then able to muster Kabaji strength (say, 4+), then all of his other stat now have a stat cap that Kabaji also have.
Here's the trick on HS, because the HS all might have high base stats (see 3rd court+some 1st stringer), Niou hit the max cap of HIS own stat, so it'll be similar to Tezuka national situation.
Niou's skill is not sufficient to return Fuji's 5th counter. As Tezuka he can return it cleanly. Yes it shows him having the Hyakuren aura but notice the hit just went past Fuji. That implies he can simply hit the 5th counter back normally because if it's supposed to apply the Hyakuren property, then Fuji should be able to hit the ball and then have it drop before the net. After all there is absolutely nothing hard about hitting the 5th counter itself (getting it over the net is the hard part).
Although Niou's base stats probably isn't bad, it is pretty clear there are people better than him overall. If he can bust people's racket normally he wouldn't need to copy Kabaji. Tezuka's skill is higher than his normal skill level. Even if you can't get the max stat of whoever you copy, Irie at total stats 22 is clearly the best person to copy so far. Irie being a low endurance/power guy also ensures you wouldn't accidentally copy any moves dangerous to yourself, since Irie's low endurance/power pretty much guaranteeds he won't have any moves that hurts himself.
Yeah, it's been great. I must admit I didn't like either Kite/Marui (as in they aren't in my top 10 favorite characters for sure) but I enjoy their match. But because I'm not their fan, I don't really care whether 1st stringer win or they win though.
The most exciting match that I'm really looking foward to is the one after this, Shuuji vs Sanada.
besides that though Tohno, Kimijima and Marui play some really normal tennis as did Ochi and Mouri. No.s 10, 8 & 7 play a far more boring tennis than Hakamada, Date, Fuwa and the Mutsu twins.
Still waiting to see the evidence that makes them better than Fuwa, Hakamada and Date.
I assume they must better, but I would like to visibly see it lol.