Miniscule in practice. Talk all you want about the Grover-tuners - the standards on the RG7321 do the job more than well. The stock pickups are a tad better - but in the end will probably more than likely end up being ripped out just as well as the 7321-stocks. Electronics - you lost me there. You're talking about wiring and stuff there?
As far as the neck goes - it's a onepiece maple as far as I can tell. As far as neckshape goes - I've played an RG1527 which had a Wizard-7. The difference between the two is tiny - apart from being a tad slicker on the low-end, there's no reason to get a mint indonesian RG7321 over a bashed-up korean RG7-whatever. This goes more on a case-by-case-basis than anything else.
Still ends up being a miniscule difference in practice. The major selling-point is the QC. The solution is just to thoroughly inspect the alternatives if possible.
First of all, they are Gotoh tuners, not Grover.
The 7620/1 had Dimarzio Blaze pickups, as far as I know. I'm not sure of the model but quite a few people I know didn't replace them. Regardless, the pickups on the Japanese models are better, whichever way you slice it.
As for wiring, yes, I'm talking about the quality of the solder work and the electronics themselves. On cheaper guitars the wiring can be good or it can be terrible. Pots and switches are cheaper. Sometimes they are not shielded too well, and so on. Hardware on the 7321 is apparently very soft. The saddles start breaking strings, and so they tend to get replaced with GraphTech ones or similar.
The RG74xx and RG76xx guitars had Wizard 7 necks which was based on the original Wizard (before Prestige). The RG1527 has a Wizard Prestige 7 neck which is thicker than that found on it's predecessors. There is a difference to both profiles, and comparing the 1527 neck to the Wizard II neck on the RG7321 will be less of a difference than comparing the Wizard II to the Wizard 7.
The 7321 is a pretty good 7 string. The 762x are amazing 7's. The 742x are good apart from the Lo-TRS bridge on the 7620. All of them can be found in good condition for not much more than the price of a brand new Indonesian 7321.
This discussion is very much like comparing the RG770 to an RG370 or something.