whilst i have own both genuine and jem-a-likes of various types, i always feel you "expect" more from YOUR jem, purely due to money and time put into it.
Now it is possible your custom Jem will be awesome and slay all Jems, it's also possible that in putting it together it ends up being a right dog.
i guess what i am saying is, a real jem,custom jem, rg550-turned-into-a-jem and god knows how many other ways there, the end result will only be as good as the putting together and finally set-up, if you have never built a guitar from scratch i'd suggest getting a cheap jackson/esp/ibanez and ripping it done, un-wiring it all, taking the saddles off the trem and really learn how to assemble and set-up a guitar (i apologize if your proficient at assembly)
Good luck with whatever you decide, and remember if you get stuck, were all only a message away
for what it's worth i have made many Jem copies over the years, alot of members have seen these, a few even own them, the bonus is you get to make YOUR Jem