"The simplest explanation might be that the cross-breeding of inbred lines
simply allows for reversion back towards a more complete wild-type genome."

Reversion is not hybrid vigor.  Hybrid vigor results when you make a cross and the F1 is superior to both parents in at least some important aspect.  Note, the mule as an example.  It is a cross between a horse and donkey.  F1s are typically large and capable of very hard work.  Many feel they are smarter than a horse as they will not work themselves to death like a horse or over drink like a horse.  Mules have a more robust and forgiving digestive tract and are more sure footed than a horse.  Yet they are not without faults.  Males are invariably sterile and females are nearly sterile in line with Haldane's rule.  Also the sex ratio is about 44% males to 56% females again in line with Haldane's rule.  The lion by tiger cross is another example where the F1 is larger than either parent by a considerable amount.  Yet, in many species crosses there is no evidence of hybrid vigor at all even if the F1s are fertile and the offspring healthy.  Even in situations where hybrid vigor is common, such as corn, such crosses do not always produce any hybrid vigor at all.  You just get F1s that look much like the dinky highly inbred parents

Reversion would be when you cross two breeds, chickens for example or equally well one of Darwin's favorite critters, pigeons, and get an offspring that looks much more like wild type than either parent.  This is simply the result of a lot of recessive mutants in both breeds being wild type in the other breed with the result that most recessives are hidden in the F1.

It is a bit of a mute point when it comes to bees.  I do not see any evidence that anyone has inbred multiple lines of bees sufficiently to reasonably expect hybrid vigor from a cross  between two lines.  Besides, with bees if such hybrid vigor did happen it would be largely lost in F2s so you would depend entirely on some queen supplier who controls the inbred lines to replace any failed or lost queens.  Just like you can not plant the seed from hybrid corn and expect to save seed money and still get a great crop.

Dick

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