I think it will fit just fine. I have Ibanez' with block, tilt and AANJ and the heels don't really come into tight contact with the case cutout, it's the neck itself which is tightly gripped because neck angles vary along with body thickness, S series are very thin and they fit, some RG's are very thick and they also fit. You may be overthinking this, relax.
You are overthinking this a little bit and here is why: the case and the guitar are made within the same period of time as opposed to decades apart.
When you buy an old Ibanez, such as an SCR220 (1999), that looks normal but everything about it is actually really weird, you end up buying 3 cases before finding one that fits. Ask me how I know.
Ibanez has simplified their cases. Old, weird guitars will fit in the W200C because anything will fit in that case. The vast majority of solid body electric guitars will fit in the M300C. The Ibanez RG8570MWNT J-Custom comes in the M300C. The RG8520SDE J-Custom comes in the M300C. The JEM's come in the M300C and down the list it goes. 27" scale length guitars (8-strings/some 7-strings/and others) come in their own cases that you only need to think about if you buy such a nonsensical guitar. In short, you bought the correct case and your RG will fit snuggly inside.
I too was concerned about this. I tried putting my RG20th in the hardshell my RG1570 came in. No bueno. But that's a 2006 guitar and I don't think Ibanez had any square heels until the RG20th the following year, and even those came in the more universal wood case.
I think the M300C is a different case than the old Prestige ones. Based on this pic it looks like it'll work.
I'll know soon I guess when the RG521 arrives.