The only info I have found about the insides of the Montage can be found over at Sand Soft Net's Montage Hardware Platform
Reading this is quite interesting, if you're technically minded, and be sure to go through the linked articles as well.
If I am reading it correctly, and assuming Paul, the site author is correct in his analysis (I have no reason to doubt him, but having no corroboration), the Montage does sound "EX5 like" after a fashion.
The EX5 had its AWM2 tone generator and then two Digital Signal Processors (DSPs) for handling the effects processing and the Analogue (AN) modelling, Physical (VL) modelling and Formulated Digital Signal Processing (FDSP).
According to Paul's analysis, the Montage has two SWP70 tone generator chips, designated as TG master and TG slave, possibly one for AWM2 and one for FM-X. That is hard to say.
What is interesting is that there is an SSP2 processor connected to the TG Slave. The SSP2 is allegedly a general purpose CPU/DSP platform used in several Yamaha models" Tyros, PSR, THR Modelling Amp, and..... Reface CP and DX
In other words the Montage (and presumably the MODX) has an AWM2 TG architecture, augmented by DSP resource (in the SSP2 chip). The question will be is how flexible is the Montage in terms of being able to reconfigure the SSP2 DSP resource to provide additional models? Has it been designed with this in mind? That is the question!
To quote from Paul's site:
But that also means to me is that if Yamaha have a CS clone in the reface series, which runs on the SSP2 chip, then why could it not be added to Montage?The SSP is Yamaha’s designated hitter when they need an odd bit of DSP work done
As I said, complete speculation, but based on this, the Montage architecture does feel "EX5 like".
There is also an interesting analysis on why the Montage FLASH memory architecture is probably fixed and non expandable, and why Yamaha go this route as opposed to the Kronos type SSD route. On reading this it all kinda makes sense to me, it's just a shame that your are stuck with a slow USB port via which to save/load data, and you of course have to live within the FLASH size constraint (much smaller than an SSD).