Hi David,
On one side, you've fallen in love with Boom Recorder while on the other you're quite passionate about audio quality (SD 302 vs XL-2 pre-amp). My sense is the signal/noise ratio on the XL-2 pre-amp is fine working in the linear range (keeping amp gain no more than two-thirds). Of course, the SD 302 is top of the line - no comparison there. But I'm a bit concerned with the audio quality of your suggested solution if you're trying to keep a high-quality audio chain. I can't imagine the audio specs of a $180 Mackie Onyx Satelite comparing with the SD722 - I suggest you closely look at the specs for the analog/digital converters.
FYI - I also enjoy using my laptop (PC) for audio recordings, sitting right next to my Marantz PMD660; call me "belt and suspenders kind of guy" - I'm not willing to lose critical audio due to technical or cockpit snafus! But I haven't found either of these solutions provide better audio quality than my XL-2s.
On the other hand, I've found the gain pots on my Alesis Multimix are awfully noisy when the amps are turned up beyond half way. When I'm using a high quality inefficient mic like my Heil
PR-40, I need to use an external pre-amp - forget the amp on the Alesis! So even while Mackie makes good stuff, we need to be careful with the configuations (sorry if I sound like I'm singing to the choir...)
Good luck, Michael