That's very cool you got your game on Live Arcade! I've been playing it a ton, getting way more than my money's worth. It's totally right up my alley.
Is it true Blazing Birds does not support online multiplayer? I haven't checked. Online play would be a great addition at some point.
I've noticed some strange stuff regarding Signum IV and saving/continuing. I don't know if it is intentional or not. Once I am against Signum, if I lose I can choose 'continue' and fight again, but if I choose to quit then next time I choose the continue option from the menu my game is gone, and I must start over. But, and this is the strange part, if I beat Signum, then later choose continue from the menu, I can battle him again.
I have not tested this extensively. I was in Normal mode when I beat Signum, and Hard when I quit. It is possible this may mean you may battle Signum IV over and over and keep upping your score, making the leaderboard into a race to the max score. But like I said I haven't tested this much, so this may not be the case.
I hope your new game does well!