As the patient once said to the doctor: "It hurts when I do this.." and the doctor replies: "Then don't do that.."
In other words..many sims fly pretty good within the 98% of normal operations, but break down at the extremes..so my guess is that if you fly the default 747 as it should be flown, it is probably OK. The same was true for most FSX add-ons (pay or otherwise)..if you put them in an extreme flight parameter, the FS physics just weren't there. X-Plane is arguable better in that regard, but not perfect.
Not necessarily, but probably usually. I mean, a paid module (one would assume) would have a development team or author trying to at least approximate the 98% flight regime (speeds, fuel flow, altitudes..that kind of stuff). But there have been free modules that are as good as payware modules without a doubt. And some payware modules that people probably shouldn't have paid money for.