Agreed, the only issue with the SE5a is the FM of the aircraft that surround it. The SE5a FM deserves to be in a study sim.
But that's the problem, the SE5a was a study case because they had a large amount of data to work off of to get the flight model precise.
As Piecost is saying, in the absence of data they have to make guesses for the large number of variables that get plugged into the FM. The guesses can possibly be adjusted to tinker with the FM, but you're talking about hundreds, or possibly thousands of variables all interacting with each other. If they have solid data like they did with the SE5a then they can get an accurate FM out of the engine. The problem with the german scouts is that there isn't enough of that data, so any FM revision would not be a refinement but instead another series of guesses of those variables in an attempt to adjust the final behavior of the plane. It would take much longer to do, on something that isn't going to give them any return, and on top of that the result would likely be another skewed FM with new issues (like, for example, a power boost to the DVa giving it the ability out turn a camel in a sustained fight). What happens when the entente pilots start to complain when the German scouts are given an edge, or display those new 'unhistorical' behaviors? Since both sides realize they can sway 777 to make an FM change, you get even more massive fights than the current complaints and there's nothing in the FM that concretely supports one side or the other. Does 777 then spend more money to swing the planes the other way? Does it take the risk of not doing so, and losing funding from one side or the other?
There's also the problem of ROF not really being a war simulator. I don't think many of the things that affected combat ability of planes are modeled, like engine wear, poor fuel, mechanical defects, the mix of engines mentioned already, etc. It makes battlefield reports, which are already often misleading and highly biased by the pilot's perspective (rather than the machine's), a difficult source to rely on.
I don't mean to say this isn't a problem, of course. I'd like to see some work done to the FMs on a number of aircraft, but I don't see how 777 can really do much about the issue without either a lot of cash (at the sacrifice of new content) or a new source of solid test data to fill in the gaps. In the meantime I don't know how starving 777 of funds is going to encourage them to fix the issue; if anything it may just lead to more rushed FMs.