Yeah, I think you got a lemon. Although I haven't flown that particular plane, RTF or ARF models these days are a crap shoot. Even from the most reputable vendors, the occasional QC slip happens. My Thunder Tiger Lazy Tiger had raving reviews but mine was really off. My ailerons hinges were so far off that one side of the aileron is two mm below the wing and the other is 5mm above it. Still flies okay but dang there's a lot of flutter! My WM EP Clipped Wing Cub has a wing twisted like a politicians. But WM kits in general are great!!!

If you run out of down elevator trim, I'd check the incidence, not all these planes are made with great precision and I've had planes that the incidence is 10 degress off and completely threw (word used correctly here :-p) off the balance. Had trouble landing the plane because it never wanted to come down! Beecause with CG at 25% of the chord, you shouldn't be experiencing anything tail heavy at all!

It is rare to see 10 oz of lead in the nose for a small ARF... did you change the design at all? You know like mount the servos in the rear or mid mount the tank? Or go with a much smaller power plant?

It sounds like you are an experienced modeler so I'm sure this doesn't add too much value to you but, sometimes the most obvious things are likely to be the easiest to overlook.

And despite prophanger's grammatical error, his comment is not that bad.