On a Miata suspension & tires give the biggest bang for the buck.
Cant say anything bad about a good rollbar...although I dont run them because I am too tall and I dont want rollbar impressions in the back of my skull.
The gutting of a Miata does remove some weight but IMHO is for hardcore guys who dont mind a loud and nasty ride. Keep in mind, nothing on a Miata weighs much so one can remove alot of crap without really removing much weight. My preference is toward forced induction, which can make Miata very enjoyable to drive and one still retain the 'comforts' if they like them.

The downside of FI is that power can be addictive and many get caught up in trying for certain HP numbers rather then a drivable car...which is a mistake! A well tuned, simple, FI setup can yeild a reliable and quick little car. Of course, all of this takes $$
The best new intro turbo kit out there right is the BEGI-S kit from Bell Engineering....about $1600-1700. FI also leads to upgrades to clutch and diff which is more $$

The suspension and tires would be the first on my list.