I agree with stoicfury. But I think there is an additional argument that the crux of idealism can't be undermined in the way you propose (whatever may be the weaknesses of Berkeley's version of idealism), even beyond the idea that everything is "theoretically observable".
In the case of "matter (with) properties beyond our perception" there is still a very concrete, untheoretical chain of perception between the so-called object and our experience of it. That might be a long, complex series including observations of readings on instruments, data on computer screens, and so on. There may (must?) also be inferential links in the chain.
But nothing in such a chain is fundamentally different than so-called ordinary perception. Yes, we use artificial instruments and complex apparatus to generate the ultimate perceptions of the "objects" of physics such as subatomic particles and waves, but how's that different in kind than, say, the apparatus of the eye and the visual pathways in the nervous system; or the use of natural or artificial light to make things visible; or the mediation of air to carry sound so we can perceive it; etc.
The trickiest part of that might be the use of inference to "see" the properties of objects such as mass, charm etc. But even in ordinary perception there have to be mental processes involved, a kind of instant inference, to "see" a fully formed object when all we really get from the "outside world" is raw sense data, with no actual object. Yet we "see" a whole tree all but instantaneously.
And often we need higher-level mental processes, a kind of inference, to "see" objects that are ordinarily familiar. I doubt if a primitive tribesman could look at, say, a cell phone and "see" anything near what we instantly perceive when one comes into view. And in fact, on first glimpse, from an unusual angle, partly obscured or under reduced light, even we might not "see" a cell phone where one is present. But as we look harder and think about what we are looking at, and compare it to what we know about various objects, it might suddenly become apparent -- hey, that's a cell phone. Even further, everybody has certainly experienced looking at so-call objects and not being able to tell even where one object ends and another begins. (
As for the need for artificial instruments or the lack thereof, I keep a jeweler's magnifying eyepiece on my desk for looking at ultra-fine print and other tiny objects that occasionally cross my path. And even then I sometimes need to think hard to fill in obscured words or properties. I've occasionally even resorted to magnifying fine print and small images with a copier/scanner so I can "see" them better. So much for the unaided senses.
Actually, if anything, modern physics has, IMHO, strengthened the case for some sort of idealism. These properties of particles that we "see" -- "color", "charm" and "spin" -- have little or no actual basis in sense data or ordinary perception. They are -- sometimes whimsical -- metaphors for "properties" that are arguably largely mental in character. "Charm" is of course, a particularly good example. Arthur Eddington, the astrophysicist who first confirmed general relativity, was a major proponent of this view.