When I first got the SDAs, I figured my first and only upgrade would be the RDO tweeters. At the time, I was running both tweeters, which meant $200 to complete the upgrade. So, I waited. Then, I disconnected the SDA tweeters and was just running the stereo ones... Which brings the upgrade cost to $100. But, that was still cost prohibitive in my mind, so I waited.
Then, I got hooked up by a friend of Carl's... and got a pair of the RDO's for $60 shipped :)
And now, I'm in non-SL2000 bliss....
As for them being the only upgrade... far from it. The crossovers were already modded when I bought them, but I have a set of rings to install, tweeter gaskets on the way, and will hopefully be able to get gaskets for the mids and PR. There's also a set of spikes to go on them, but I'm not sure how best to accomplish that, since they're not set up from the factory for spikes. I also need to brace the rear baffle on both, as I have one that's rattling already. A piece of quarter-round and some wood glue should fix that.