RE: Dihedral?
In something like the LT40 it adds some self leveling force for newbies. If you're comfy with flying then taking it out is all good. It'll tend to be a lot slower to level the wings or may stay banked until you level them for it. It will not (should not) become unstable and try to spin but you'll need to make corrections with the aileron more often.
Inverted flight will be easier.
If you find that it does want to tighten up in steeper turns and require constant aileron correction to hold the bank then remove about 1/2 to 3/4 of an inch off the top of the fin and rudder. Dihedral, vertical tail area, tail moment arm and CG position are all factors that determine what your spiral stability is like. removing dihedral is one way to make it more spirally unstable and removing a little bit of tail area to compensate will restore the balance.