Quick observation: my previous post assumed that one upgrade is applied only once. But that having been said, even if the upgrade can be applied more than once, it is still dependent upon the strength upgrade purchased, not the amount of force pips applied, so it still tops out at 4 maximum...
Nope. According to Sam (and I included the Strength Upgrade to Move as a specific example in my question), the only existing limit is how many Force Points you spend to trigger the Strength Upgrade.
So if a PC has purchases two Strength Upgrades, they could trigger it three times (assuming they generate enough Force Points in the first place) to affect a Silhouette 6 object. Granted, you're talking a fairly powerful Force-user (Force Rating 3 at least) in order to pull this off with any kind of regularity, but the potential is there, especially once Force & Destiny enters circulation.
I see what you mean in terms of "increase"; I was (and am) reading it that it tops out at silhouette 4 in the same way that armour doesn't stack. This probably should be FAQ'd at some point. I'm not saying force users shouldn't be able to throw a star destroyer around, but even Yoda needed to focus to move a silhouette 3 object (the X-wing). However, I see this kind of power being more on the level of a "signature ability" rather than in the basic power.
Here's another thought about how this isn't necessarily broken. Assuming that your new reading is correct, a force user is capable of triggering the force move power three times to upgrade to silhouette 6. The object itself has to be within short (personal) range for the basic move power to be triggered. By definition, a silhouette 6 object (a Nebulon-B Frigate) occupies much more than short range by virtue of its volume. It's 300 meters long, which I would classify as long range from one end to the other. As such, the force user would need to also boost the range of the power just to get it to have the entirety of a Silhouette 6 object fall within long range. That's less force pips available to a user for doing the strength upgrade...