I had a Borla on a previous vehicle and liked it very much. Good sound and great fitment. I'm steering clear of it on my 04 because of the crossover. I like dual exhaust but the photos I've seen of the crossover suggest that it would compromise clearance.
I had a Gibson on a F-250 and it was OK after I installed it but installation was a bitch. Fitment poor.
Flowmaster: Seems like I read a lot of threads on the Tundra board that talked about this system being so big that it didn't provide enough backpressure. These posts all had to do with the V-8. Guys were pulling them and putting back on their OEMs.
Magnaflow: I guess this is what I'll get esp if I can get it for the ~400 bucks from performancepeddler. I sure don't like to hear about drone 1500-2200. As you all know, the V-8 hits a harmonic at 1800 and vibrates there as it is. Well, maybe a bigger final drive ratio after awhile to raise the cruise rpm a little (and get more lowend torque).
While I'm at it, I'll say that the most noticable performance enhancements I've done on trucks (in order biggest to least) are:
Raise drive ratio
Headers (w/cat back)
Larger throttle body (w K&N conical filter);
never used a chip
larger tires lose torque (of course)
throttle body risers are worthless
haven't tried different ignitions but "better" plug wires don't do anything
I actually liked Splitfires on previous vehicles but will stick w the NGK iridiums on this truck
Anyway, my two cents.
Bob