Well . . . BBs are good opamps. You can't generalize about them though, because in the BB line TI probably makes 50 different opamps. The 2134 is a solid audio performer, nothing extremely sexy, but affordable, proven and stable. I have them either stock or modded in most of my gear.
Having said that, there is a lot of sales talk in that eBay listing. Now, I'm not gonna be the pot calling the kettle black, so I ain't going line by line. Suffice to say that the sound of a pre, especially a hybrid tube design, is not all opamp. the 2134 will be a big improvement from 4580, that I can guarantee. But I don't know the Behri's circuit to say if that makes a miraculous difference.
For example, the amount of gain is not a function of the opamp, that is set by the components in the circuit around the opamp. So if gain was increased, that is not a function of an opamp swap. Now CLEAN gain may have increased.
But I can also tell from the Behri's literature that there have to be other considerations. Behri says that the input stage is discrete, that could be a significant factor in the performance of the box. Or not, it's hard to say without digging in.
In any major mod, you've got to look at the power supply. Especially in Behri's case, where their gear is not noted for reliability.
If all of those pieces were considered in the mod, it could easily be worth the $100+ cost of the mod. If it's just an opamp swap . . . I am not sure, but I do suspect that the claim that no pre under $800 will compare is sales talk, not proven fact.
I have the ART Digital MPA, which is the Gold with a converter onboard. I have been using it almost daily for over a year, it's a solid versatile preamp. Nothing too glamorous, but it does what I need it to do. Without having used the modded Behri, I know from the switchable plate voltage on the ART that I'd really rather have full plate voltage.