Is an external amp needed?

[Dase]

I know that there are benefits for having an external amp. I know that they can make a difference, especially if the speakers in one's setup have low impendance (4ohm) or are not very efficient (<90db) or if the AVR is lower power rated and has preouts etc.

What I am curious about though is a case where the speakers are efficient. Or where the AVR does a respectable job. Combinations of both. Take my humble setup for example. My speakers are 91db efficient and my AVR has been reviewed/tested to be able to push around 110 watts per channel with
.1% thd all channels driven .

So according some charts posted here in relationship to AVR power and speaker sensitivity into SPL, my setup can do:

Watts - SPL
1 - 91
2 - 94
4 - 97
8 - 100
16 - 103
32 - 106
64 - 109
110 - 110-111?

There are many members here who have added an Emotiva amp to their setups and are quite happy with them, but looking at the same type of table in relationship to, say an XPA-3 for example (200 watts x3), all it would add to the above is:

Watts - SPL
128 - 112
200 - 114?

So as an example, I would be paying $600 for 3db of headroom. This doesn't seem really worth it to me, but I may be missing the whole picture, hence this thread. Some people here have speakers with a sensitivity rating of 96db or higher, is an amp necessary in this case?

I'm not trying to open a can of worms about different amp manufacturers or AVR manufacturers. Just trying to break it down somewhat into power, sensitivity, and SPL... price vs. performance... is it really worth the $$$, etc.

Please educate/advise . I like to learn . Thanks!