A TL can never be as good as a reflex because it requires damping material to quiet the resonant pipe sounds, which drastically reduces efficiency. In addition, once a TL is built, the length of the line cannot be changed to optimize tuning. A TL will rapidly go out of tune as the drivers break in. The only real advantage TL's have is that the enclosure is stiffer due to all the wood required to build the thing.
Really? OK, some general points. For a start, what does 'as good as' mean? Until you properly define it, and eradicate any erroneous assumptions, it doesn't mean anything. Which brings us to the next point. What do you mean by TL? It's become a catch-all phrase which can be and regularly is used to describe cabinets that are almost the functional opposite of each other.
If the term is applied in its strictest sense, a 'pure' acoustic transmission line is a pipe that is stuffed to provide the flattest possible impedance load. Period. End of story. No other considerations. Whether you agree with the principle behind it (viz. whether or not you believe creating the flattest impedance load is useful) is irrelevant: that's what it is (and completely dissimilar to a BR -they would naturally be unlikely to be used for the same purpose). The vast majority of boxes described as TLs do not conform to this; most deliberately use the pipe fundamental to provide some LF output, while a modest quantity of stuffing is applied along with careful driver offset in the line to eliminate the unwanted harmonic modes. This does
not mean that such cabinets cannot produce useful LF gain. If they do not, the designer was either incompetent, or somebody enforced a compromised variation of the box for xyz reason.
As for driver break in, as Paul said, this should be done
before a box is designed, whatever the cabinet may happen to be. You may want to factor the amplifier output impedance into that if relevant, room acoustics &c. too. Since T/S values obtained also tend to vary depending on the measurement conditions, this ideally needs to be accounted for also; again, this applies to any enclosure type.