I'm not sure how to approach something like that. "Which cars would give a similar experience to the Roadster?" Similar in what way? As you noted, there is no really direct competitor.
Small, open-top sports cars would include: Lotus Elise, Mazda Miata, Pontiac Solstice and the upcoming Dodge Demon. None of them would come close to the Roadster's acceleration. (Bolting on a supercharger might get you closer, but at the cost of your warranty and any semblance of long-term reliability.) Only the Elise would provide exotic looks and world-class mid-engine handling like the Roadster.
The Miata is interesting because it's relatively lightweight, relatively good handling (as front-engine cars go), relatively inexpensive (half the cost of an Elise). . . It's been a huge success because it strikes a good balance for most people, and because it's affordable. Also it's reliable and easy to get serviced when compared to "exotics". But it's quite common, homely and underpowered compared with the class of cars we really should be considering, and it won't grab attention when you are cruising down the street. People won't be whipping out their cell phones to take your picture.
If a permanent hard top doesn't bother you, then the Lotus Exige would come closer to the Roadster in performance. . . But it's really a track car. The suspension is rock-hard, it's very awkward to crawl in and out, the trunk space is even smaller than the Roadster, rear visibility is nil -- it's far less practical as a daily driver.
The Chevy Corvette can't be discounted. It delivers a lot of bang for the buck, and is a very fast car. About the only thing it really gives up to the "exotics" is it's not-so-exotic looks and relative lack of "exclusivity". Again, no cell phone photos.
If you want it all: killer looks, exotic "exclusivity", blistering performance. . . Then I'd suggest a Lamborghini Gallardo. There's even an open-topped Gallardo Spyder available. However. . . According to Wikipedia, the Gallardo runs $186,250 - $222,800. So, you're looking at just around double the sticker price of a Tesla Roadster, and you're still burning premium gas, and the maintenance costs are hair-raising.
I have to slip in a favorable mention of my Lotus Esprit V8. It may not be as exotic looking or as fast as a Gallardo, but it's pretty darn exotic and pretty darn fast. Lotus stopped making them in 2004, but you could easily find one on eBay.