As someone working on a similar (somewhat less ambitious, but still definitely not a BDR) L3 project, I'd like to shed a bit of light on why people might decide to fly high and fast for a cert flight. In terms of the practical considerations, you always have to start with cost. A larger rocket is more expensive, because it takes more raw materials to make. I got my entire airframe for about $300 (motor mount, airframe tubes, couplers, centering rings, and a sheet of fiberglass to cut my own fins). A similar 6" airframe would cost double that, at least. Smaller (and thus, lighter) rockets also don't need as large a parachute, or as strong of recovery harnesses/attachment points, which also saves money. As a college student, I'm already spending basically my whole 'fun' budget (plus some) for this year on my L3, and making it larger just wouldn't be financially feasible, even if I wouldn't want to then build a high-performance rocket later.
Let's talk about safety and experience. I'd be surprised if the safety incident rate for high-performance models (not including research motors) was significantly higher than for 'low and slow' rockets, since the vast majority of failures are recovery failures, and the recovery subsystem would be basically identical between high performance and 'low and slow' rockets (see the NAR's 2005 special committee report). Yes, it's more likely that you don't find the rocket, but the extra $150 you might spend to install a tracker in the rocket to mitigate that risk is still significantly less than the cost difference of a larger rocket. As for airframe failures, yes, they're going to be more common in a high performance rocket, but the risk is still very small, and can be mitigated by a good understanding of the flight performance, which is one of the few things that I've found student groups to be significantly better at than a lot of the rocketry community (seriously, most rocketeers, even experienced HPR fliers, haven't done fin flutter analysis, FEA, CFD, and thermal analysis before).
So here's my advice - you should know exactly what's going to happen with any rocket you're going to fly before you push the launch button. The vast majority of certifications that I've seen fail have been because of inexperience. If you don't know what you're doing, ask. Also, make sure to take a step back when that happens and make sure you haven't gotten in over your head. But if you're comfortable that you know how everything is going to perform, go for it. That goes for both high performance cert flights as well as for a 'low and slow' project. Best of luck on your cert! Also, for your fin attachment - is there a reason you want to go with welding or brazing rather than just a mechanical attachment (I'm thinking L-brackets to hold the fins to the airframe, or maybe something akin to the max-q fin cans that Binder Design sells)? I haven't done anything with metallic fins, and am curious about your design process.
