You're giving falcon 1 too much credit, it was more like 200kg to sun synchronous orbit in real life.
As it is, the in this thread here should be able to carry at least 10% of its mass in payload if you analyse its components.
A 5t payload is actually a bit on the light side for it.
I'd specify a smaller payload if that's what this rocket was actually only capable of, via less fuel and more ballast. But not less than it ought to be able to carry in real life.
And it would indeed be more accurate that way, except for the fact that the SFS solar system is 1:20 scale... So do you scale the payload mass up to compensate?
I'd expect an SFS falcon 1 to carry 200kg... in IRIS.