I don't think it would have been that big of a change. Is being able to turn invisible by projecting the background on yourself that game changing? It is not like the Suke Suke no mi, this kind of invisibility should be ineffective against any good CoO user. The exoskeleton and the stat boosts he received on Onigashima are basically identical to the RS ones: sturdier and fire resistant body, greater strength behind his kicks. The cape use as a shield is so situational it wouldn't have changed much imo.
I'm also a fan of the RS design and, for those who aren't, let's not forget Sanji wanted to ask Franky and Usopp to modify its design. The idea of a suit-like RS 2.0 with the stylish gloves he wore on Onigashima as canister would have worked great.
I believe he went for the exoskeleton path to make everything simpler and distinguish Sanji from Franky the tech dude. The fact the transformation started the moment he wore the RS for the third time is extremely random, I still hope it has something to do with King AND the RS. Sanji having some lunarian blood due to Sora's drug could explain the fire and the timing of his lineage factor modification flaring up, especially with the RS acting as a catalyst