Anyone have any experience with a Ryoba japanese saw?

[Demit]

I ordered a wall cabinet along with my Ultracraft vanity for my impending bathroom reno. It's the standard 12" deep (13" with door). I plan to recess it between studs in the wall behind the toilet. What I'd like to do is reduce the depth of the cabinet to 7" so that it will only jut out from the wall 3" (well, 4" with door). It's 15x30, and constructed of 5/8" melamine-on-particleboard.

The way I envision this being done is: take off the door and, without taking the box apart, slice the box lengthwise into two parts, toward the back. Then cut again, cutting off 5" from the larger half. Clean up the cuts with my palm sander then glue the two sections back together. The join would only be visible from the inside of the cabinet, if at all, which wouldn't bother me.

Is this something I can do with a Ryoba japanese handsaw? I've read that it makes very clean cuts and the action is easier to do. Is it something I could do myself? I'm not a carpenter. I'm a graphic designer with pretty good hand control and hand-eye coordination. If a carpenter (who is not a cabinetmaker) did it, what would he use?

Is there a better way to approach what I want to do? I would take the back off and just make one cut, then put the back back on, but I can't see how it is joined. The website says the cabinet box is "dowel-and-glue joinery with machine-fitted back panel."

Even if I end up not doing this, for fear of ruining the cabinet altogether, could someone with more experience tell me how they might approach it?