Theory:
It wasn't the RS but seeing Nami naked that triggered his exoskeleton. His blood was racing so fast that it caused the drug modification to ignite and Oda is using the RS as red herring.
If it was the RS, then it should have triggered his exoskeleton the first time he wore it.
His PUs are coming from his emotions.
Yeah, Sanji's human emotions plays a big role on who he is and even in his powers since ever. Oda is surely going to make it more relevant than it seems (i mean, its kinda happening already)
I said before that i think Sanji will be able to boost the Exoskeleton by himself, like he does with Devil Leg. Judge despises emotions so much that they will be one of the reasons of why Sanji is much stronger (as it is already, but will be even more)
Another interesting question is:
Is the Exoskeleton really that superior to CoA? Thats how chapter 1029 presented it to us against Queen, but is this something his siblings can do, or only Sanji himself?
Blades, and overall slashing/piercing weapons are already tough to block with CoA, Sanji using CoA now will be disgustingly powerful
Even if your own haki is already superior to your opponent's CoA