If you want to go extreme, and have the money to play that game, the usual options are available. Find someone to custom craft you a carbon hood, or buy one if available (highly doubt at this time). What you say is true, and the hood weighs a considerable amount, so going carbon should drop weight exponentially. Also try carbon fiber options for other panels, or the likely needlessly heavy doors.
I see you've gone with lighter rims, in tune with that, the spare tire can go entirely, and if in the case of GT (or Grand Touring is it elsewhere? Top Trim?) drop the sub in place of the spare wheel.
You could look into lighter options for front seats. There are universal front seats (Assuming, once again, that no aftermarket options are available directly for the CX3) that would also weigh far less than the stocks do. You won't have much luck with the back ones, other than removing them.
There's a whole host of smaller weight reduction options too. For example, an exhaust system made of lighter materials. Even with aftermarket swaps of engine components, find lighter options.
Of course, all of this is entirely under the assumption you have money to blow. In which case, kudos to you, I take pleasure in seeing creativity in others. Or Id suggest, if money isn't an issue (and weight and power is important), trade your CX3 in for an Evoque? (reliability aside)
Realistically, considering I'm sure Mazda have already gone out of their way to save as much weight as possible (I mean, they are the makers of the Miata), only serious option is the stock hood for a carbon one. That is the largest weight loss that could be had.
PS - I don't mean to be an ass with that last bit about the trade in, it was a serious option. As I've mentioned in other threads, I consider myself to be at the forefront for modifying the CX3 beyond Mazda's original intentions.