What you have done looks good and I love a painted bottom like the top side. I also like the same color firewall and engine compartment. I was just stating the fact about the transition line. I have not heard about the debate about if the bottom was painted. I have heard about the primer bottom and the Camaro assembly process kinds of backs it up.
You did what you wanted to do and it looks good. That is all that it matters. If it was mine I would also do the same to the firewall and wheel wells in the engine compartment. It would look awesome.
