Small arms is not my area of expertise, however im pretty sure an infantry section equiped with M16A4's has no more "Firepower" than one equipped with L85A2's.  So Im not really sure what your question actually means!!!!  They both fire NATO Standard 5.56mm ammunition both have the same barrel length (to within a few mm) so therfore have almost identical ballistic performance.  Its often stated however that the L85A2 is the more accurate of he two.  Im not sure how true that is.

 

Also, the M16 is NOT a carbine.  By definition a "Carbine" is a shorter barreled version of a standard rifle.  The M4 series is a carbine however, as is the L22A2..

 

If you are really just wanting to know which is the better rifle I would saw ther is not a lot in it.  Ergonomically I prefer teh bullpup configuration.  But thats just personal choice.  Where the L85A2 falls down is that it cannot be fired by a left hander, for me thats a real problem as I am also left eye dominant.

Not sure what you mean - the SA-80A2 has a longer barrel length than the M4 etc, has a good track record of reliability, fires largely the same ammunition and has similar rates of fire. Where's this extra firepower coming from?

 

In a British infantry section as with many others, the main base of fire is from the Minmini and GPMGs anyway - the rifles tend to be secondary by comparison.