What would be the effect of doubling the egg in this bread?
While it might seem like a drastic step to double the amount of egg in your recipe, in fact, you are not adding that much more egg relative to the amounts of your other ingredients.  

A whole large egg weighs about 50 grams and is roughly 75% water so that extra half egg will contribute just under 20 grams of additional liquid to your recipe - you will need to compensate for that additional liquid either by reducing your water contribution accordingly or by adding a little extra flour (like 30-ish grams).

That means you are only adding an additional 5 grams of protein and fat from the egg by including that extra half.  You will probably notice a slightly more tender or soft texture in your finished loaf from the slight increase in egg.  

Since texture and flavor are so subjective and difficult to describe, the best way I can think of to communicate the possible change is to ask you to think about Challah bread.  Challah bread owes much of its characteristic texture, color, and flavor to the inclusion of a lot of egg in the dough.  While adding that extra half egg won't bring your recipe into Challah territory, it will advance your bread's texture, color, and flavor along the spectrum in the Challah direction.