In a real driver you won't notice the difference between metal and non. The LPG is seriously great off axis.
Why are you only looking at SB stuff? I realize there is a misapplied boner on diyma, but they aren't anything special. I've been completely underwhelmed by all of their stuff. Sure it is priced interesting, but the little savings rarely makes up for the harmonic distortion or other general lack of clarity in their response.
You can ignore the on axis response on plots completely. You will NEVER sit on axis. A great response on axis will therefore be completely pointless. The only thing you care about is the off. Having the 30 & 60 curves be similar would be helpful though as your left and right tweeters will be at severely different angles. I would also ignore a single point on the curve as that is easily adjusted with eq, but instead focus on the overall response. The less eq tapering you need to do the better.
Large format in this case may not buy you much. Fundamentally a 3/4" tweeter will do what you need better than a 1" tweeter, but it will do this by sacrificing the bottom end. Conveniently you can choose a mid to fill in that bottom end, but having some flexibility is nice. I'd pick FR over flexibility though in this particular installation.