I have the RIPMAX RC Simulator (I paid $10 from Tower so I wasn't expecting 100% realism but I was hoping that it would be close enough to learn something valuable...).

I underpowered the virtual trainer so I'd have an idea of how an underpowered plane might fly. This is what happened. Zero wind. Approximate speeds on a flat course.

Virtual Trainer: 5 lbs. With:
MDS 18 Pro & 8 x4.5 prop: 53 MPH
MDS 28 Pro & 9 x 6 prop : 77 MPH !!!
MDS 40 Pro & 10 x 6 prop: 54 MPH
same as above with 11 x 6 prop 60 MPH.

Is it possible that a real 5 lb trainer would go faster with a smaller engine like a .28 and even fly with an .18 engine?

the plane would still fly JUST i think with a long enough runway with the 28. i don't think the 18 would even move it on the runway.

that sim is pretty poor mate, but for what you paid for it then i suppose it's usefull.

cheers