[Sputter]

You can toss coin at and make marginal at best gains in torgue. Noticeable gains can't be bought cheap. It's just not that kind of car.

Sheading wieght is likely the cheapest and easiest. Although you've already added about 100lbs of wieght with that top. (it's almost like driving around with a permanent passenger)

Swaping motors isn't cheap. (to a 3.2 or larger)
Having Flat6 do their magic and increasing the liters won't be cheap either but it's an interesting way to go.

Adding a turbo kit or supercharger sounds like fun but the added stress on the engine parts is questionable long term. Guys have done it, see some threads here, they can fill you in on costs etc.

I know it's not what you were looking for but others here can likely offer a different opinion and ways to get some extra.