Some good advice here and I will agree w/ a good amount of it.
First and foremost, always check out where you think you might train before committing to it. Even if there one golden, superior martial art, it wouldn't mean a thing if the master/sifu/instructor/etc... was crap and if the mindsets of the other students/trainees are wrong.
Second, find what works best for you.
Third, don't fall into the 'my martial art is the best' mind set. There are always elements of other martial arts that you can fold into your training/techniques to strengthen your offense and defensive capabilities. No one martial art is w/o flaw.
I've trained muay Thai for several years but stopped short of fighting amateur. I'd train w/ some of the fighters, but didn't feel I could commit enough time to train right to step into the ring. I've also tried "full contact" kickboxing, which is the same as Western, and it confirmed my distaste for it...partially because of my adherence to muay Thai but primarily because, as others have said, what I'll call 'regular' kickboxing is more sport. Also, I would disagree that muay Thai is not applicable to self defense on the street. There are many techniques from fighting in the clinch that translate well to street fighting. Combined w/ the OPs boxing background, I feel that would work well.
Likewise, I don't consider BJJ to be 100% effective in the street. I've trained in Pancrase and informally trained w/ friends who train in BJJ as well as BJJ instructors so I am not 100% ignorant. Before anyone tries to flame for the former statement, note that I do not consider any single martial art to be 100% effective. There are aspects of BJJ/LL/Pancrase/etc... that put you at great risk. Standing techniques fair better than ground techniques. Even still, I am always cautious that my opponent may be carrying a blade so I prefer to not stay tied up for long. BJJ/LL/Pancrase do have very useful techniques for controlling your opponents center/balance, though, and certainly any escape from a mounted position is well worth training. There are also some useful escapes from other common holds like headlocks and front chokes. I just don't see any validity to trying a helicopter armbar in a bar fight. lol (although that would be pretty frickin' cool).
Krav Maga rolls up various techniques to apply to different 'real world' defensive and offensive situations. I've never tried it, but people I know that have have liked it.
OP, FWIW, I think boxing is very worthwhile for self defense. Realistically, most people are not going to come at you w/ a roundhouse or tornado kick (although the latter is pretty awesome) in a street fight. If they kick, it's because they'll try to kick you in the balls (knowing how to catch a kick and use it to off-balance is worth knowing, though). I have always found the straight punch to be my friend given that most people favor the wide, looping gorilla punch. I also love a good shovel hook. Your assumed use of angles and knowing how to slip a punch also gives you an advantage over most.
All that said, I would actually recommend trying Krav Maga (even though that's largely uninformed advice since I haven't dabbled in it myself) or, to the chagrin of others, find a good school that trains MMA. Again, just my opinion, but I don't know that training in one particular martial art will get you what you want (if I understand it correctly). If just going for real world self-defense, it would make sense to be well rounded in different situations. To me, unless you specifically want to train in something directly, you're better suited to something/somewhere that can expose you to various disciplines/situations. A proper place that can do that is a place that can teach you either an all encompassing art or that offers multiple disciplines in one spot. Since you're not looking for a black belt, a "MMA school" can do that. I've trained at a couple places that trained MMA fighters (amateur and professional) and consider them to be worth that much. It all goes back to quality of instruction, though. I think aspects of clinch fighting (a la muay Thai) as well as some of the positional and control aspects of some grappling arts would work well for you given your boxing background. With some of the grappling/ground arts, though, note that striking is not allowed and therefore, in a self defense or street fight situation, can leave you open. If you do consider a MMA school, visiting and observing the instructors and students is very important. Ever since UFC gained popularity in the US, and especially after Lesnar signed w/ them, the # of schools increased ten fold. That made it harder to find good places to train. I would start looking in the Training subforum on Sherdog. Most of those people are competent and have actually trained to some degree. Most of the idiots tend to stay in the Fight Discussion section. Unless you want to lose brain cells, avoid the latter.
As far as MMA being WT, I disagree but can understand the viewpoint. Personally, I put a lot of blame on how it's marketing for how it is portrayed and generally perceived in the US. UFC, when owned by SEC, took the wrong approach and suffered for it. Fast forwarding to Zuffa, I see the same thing. It is a money market now w/ a generally uneducated public audience, poor marketing (as a sport), and a large audience going to get drunk & watch people 'beat each other up.'
Such ends my dissertation. Flame suit on.
