Since IR's have come up quite a bit and I'm a big fan of them, here's a few suggestions and my .02 on it.
In the example of needing something played through a car stereo, you could always create an IR of not just the car interior, but the impulse being played through the car stereo, thus getting you a much closer result than just trying to capture the acoustics of the car.
An IR is simply the mathematical measurement of the dynamics of an "environment". Impulses don't just simply capture the reverb, but how a specific sound reacts in any variable of "environments" and the software then creates a convolution algorithm from that IR, whether that IR be from a physical space or the limitations and parameters of a specific signal passing through different stages of a piece of hardware, cables and it's settings. Any recording you ever make is essentially an IR of that source signal going through the various stages of processing you imparted upon it. It's now represented as a waveform in a digital file or on tape or whatever the medium. We call it a recording, but in reality it's actually an archive of an impulse response. We just typically don't view it on that much of a microscopic detailed level.
Example: Focusrite's Liquid Mix is essentially just Impulses being recorded through gear with incremental settings so that they can essentially "clone" the gear by creating a range of IR's that it calculates between. Thus supposedly giving you as close as an accurate representation of what that piece of gear is able to do to a signal. In essence, making it much more complex than the single snapshot style of Waves' Q-Clone or a simple Convolution Reverb with minimal parameters.
All the Convolution IR plug-in is doing is imparting the dynamic characteristics of an "environment" on the input sound based off of the dynamics of the IR that's loaded into it. Which is exactly like what we would be doing by "worldizing" something. We're just doing it in the analog realm as opposed to archiving that analog realm for future use as an IR (which is exactly what AudioEase's Speakerphone is). It's really not too far off from how a vocoder works (in a very loose concept).
Whether the results are realistic and usable depends entirely on the quality of the IR made. The only difference is that the IR may offer more or less dynamic control depending on what the IR is made of. You will have less control with an IR of a piece of gear that has countless setting variations than you would of a impulse being recorded in a simple environment that you can do nothing to change besides mic and source placement when creating your IR.
So, to save you time in the future if you happen to need to worldize sounds coming through a car radio a lot. You could maybe spend a day doing cars with different grade stereo's and cabin sizes to create a worldized IR car stereo library to choose from. You could also do this with cellphones, various TV's, radios and so on (Or you could buy Speakerphone, but that takes the fun and a bit of the originality out of it in my opinion). Personally, when making my own IR's I would use various sorts of Impulse triggers from very short white noise bursts to full test tone sweeps to get some variation.
Hope that wasn't overly complex or wordy and it helps. Mostly, have fun with it.