Let's take the example of a 7.62x51mm gun
20" is 780m/s and 16" is 750m/s.
A decrease of 30m/s would go unnoticed in most cases and start to show up only when you start engaging targets of a 20" 'Designate Marksmen Rifle' instead of the intended range of a 16" 'rifle'.
As for ergonomics, the rule of thumb for rough estimation max length is [barrel]+[7" receiver]+[7" stock]
> 20" bullpup would be 20+7+0 = 27
> 16" conventional would be 16+7+7= 30*
*most conventional rifles would have a folding stock so if you want then you can reduce the length to 16+7+0 = 23"; folding stock is extremely rare in bullpups.
So...naah; no real benefits of a bullpup despite having a longer barrel
The only scenario, at least in my knowledge where a bullpup is clearly better than a conventional rifle is when there's an extreme constraint on length. In that case the barrel of a conventional rifle would get so short that every trigger pull would be a flashbang.