Miggs - Where do you plan to install it?? Remember, that nice pretty screen etween the seats that looks like it covers a sub or speaker has approx 1 1/2 INCHES of clearance to the CHASSIS of the car (as I learned when mine was being done). The stock sub, which is only about 2", is housed vertically inside the wall between the 2 seats, just in front of where the top goes down. I forget if yours has the rollover protection bars. My 8" Kicker CVR sub (had a solo baric but it was defective and blew without reason the first night so was changed with Kicker's guidance to the CVR) BOOMS
with the top up for the bass to bounce off, and still kicks really hard with the top down since I have the wattage to power it. As Bull says, and my local guru did as well, you of course never can get the same level of boom without a roof to bounce sound off of, but with the right stuff you can do well. My sub is a Kicker since Alpine doens't make smaller than a 10" sub. The tech who actually built my box worked his tail off to make it happen in such small space, yet another attraction for the Roadster's new Papi...
Anyway, 8" is for sure as big as you can go without just taking over your whole trunk (which I did not want to do as we take the Roadster on trips). So I went with letting my guy put the Alpiine power amp for the sub on the right side of my trunk, bolted in, and hardwired to the battery and grounded, etc. I have a crappy phone camera pic I took for you months ago that I post here now. I also am reposting here the distance shot of the sub-box between the seats. My more recent mods included an i-link which allows the Alpine head unit to control the sub directly, not just via the EQ. This has been a great upgrade as despite power, crossover limiters, etc, there are just some things an 8" sub cannot do (but not MANY...
Bottom line, I changed out the head unit, all the low and mid speakers with Alpine and slightly bigger ones my guy was able to make fit (the stock ones on my 2002 model were laughably tiny, yours must be absolutely micro since the sound system got better each year) and the sub set up and power amp and later the i-link and i-Pod interface to the glove box. The tweets screamed and were fine. Now that there is enough RMS power for the regular speakers and juice for the sub, the thing RIPS, as Prof can tell you as he's heard Metallica on it with the top down.
There has to be a decent inexpensive option and I think you are goping the right way with one of the small, self contained sub packages and maybe just changing out the head unit and a few main speakers. The work is what made it really pricey, the parts weren't that bad at all (though the bastard soaked me on them but did such a good job then and gave me a break on future upgrades etc. so I can't really complain now).
Sounds like you are planning this well. I hope this helps some and of course, malecow remains the authority on all things of this variety.
Good luck bro'.