As you said Hisoka playing around makes things a bit different. But with Kastro's feats and Gon and Killua's growth. I don't see how they could be beaten, they have more combat experience than before, are used to use Gyo by now, have fitting Nen abilities etc.
I don't think it would be particularly hard for them to beat him as they are now. Gon's amount of raw power is no joke, definitely above Kastro's and they are both Enhancers.
If Kastro needs his ability to be hidden to win, then he isn't that great of a fighter to begin with.
I must respectfully and yet strongly disagree on this one.
Nen users especially thrive on hiding their abilities. Just by following Hisoka(the more outspoken character), we see that even though he likes to strut around bare naked even with his nen, he still has tricks up his sleeve. The Ryodan's might not shy away from hiding their nen power levels, but their skills are especially unknown, even to their own.
With that said, Kastro is clearly one of the higher level nen users in which I don't think Gon and Killua are there yet. It took Hisoka 2 limbs to produce an assumption, an "assumption", to how Kastro's abilities work. If he guessed it wrong, Hisoka would've died that night. That means 2 things:
Kastro has high enough power level to damage and possibly kill Hisoka.
Kastro has a skill level that may not have been considered master, yet(give it a few more years), but it should be considered lethal to unprepared fighters.
Conclusion: In real nen battles, Kastro would've killed Hisoka right then and there. So Hisoka's win, was mainly attributed to the history between the two and the circumstances they were both uniquely tied to. So we could say, Hisoka might have been just playing, but in his mind that was the only way he could have figured it out. It was a G R E A T gamble, in which paid off nicely. Kastro never wanted to kill Hisoka, he wanted to humiliate him. That's what Hisoka was going for, if they never had that personal inclination, then the fight would've ended completely different. And I for one, am willing to bet Kastro would've come out the victor. Yes, hisoka overall was the better fighter and still always will be, but he's not perfect, one of these days he's antiques would cost him(Chrollo). But it doesn't mean Kastro, without those special circumstances would've lost.
Okay, I realize I've shot myself in the foot by saying if they hadn't known each other, yadda yadda.. Since it was Hisoka who inspired the doppleganger ability in the first place. But this is not Hisoka vs Kastro, and this is an imaginary fight between that level Kastro and current level Gon and Killua, in which I would really wager that they would lose indefinitely.
Here's my analysis on Kastro as a nen user:
Physical prowess on par with Hisoka maybe stronger(Enhancer: How else would Hisoka win against Tiger Claw)
Nen prowess on par with Hisoka maybe stronger(Enhancer: again)
Fighting IQ below Hisoka, but just a bit.(Hisoka thought Kastro as one of his edibles, up until he messed up his own potential)
All in all Hisoka doesn't take "STRONGER" opponents, he takes "SMARTER" opponents. If he did, he would've challenged Uvon right then and there.
He wants the mental challenge, and for him to consider Kastro, just goes to show how capable Kastro really is, even if he jobbed just to make Togashi's point.(Togashi likes to throw away really strong characters just to make a point, pfft)