That's not bad. I'd expected a lot more than 100lbs increase. Reminded me of the guys who used to shoehorn Prelude H22's into CRX's. Had to carry $200 worth of groceries in the back just so the rear tires would touch the ground.
The site I referenced earlier in this thread makes a tubular k-member which shaves 10 lbs off the front (They also make a version without motor mounts, so you can drop in any engine you want). If you went with aluminum coil-overs, tubular control arms and aftermarket brakes, you could save even more weight. The turbo 4 route is an option, if that's your bag. One of those JDM-spec EVO motors and a monster turbo would work pretty well. I like the idea of the Subaru flat 4, but it may be even wider than a V8.
The first V8 Miata I saw was in the very late '90s. You could see the long tube headers and big oil pan underneath. He was going for the Japanese "Shelby Cobra" look. Blue with white stripes, Halibrands and even the dual fans in the grill. It looked and sounded wicked and ever since, I have been thinking of building one.
Of course it won't be cheap when all is said and done. But it will be scary fast and I won't have to wear a hat and dark glasses when driving it in public. If you wanted to do it on the cheap, you probably could build it for less than a brand new one.
As for the OP, it seems unlikely that the typical Miata owner would be interested in an assault rifle. But I wish you luck.