i have some experience with 5 paladins group. It was my first lvl 80 group.
No great xp but i played this group in last 2/3 months.
pve and pvp.
My paladins have actually my best gear of all my "army".
Consider my best gear is not great gear (basically all pre raid gear+basic pvp gear).
I done many testing focused on Pvp...and at the end gone for dual spec for all, for save some gold for my future crazy specs.

All retri:
it
s fun in bgs. AV is best bg for me. 15 men bgs (wsg, arathi, eos)
depends by other players. But in general it s funny too. now i have
22/23k hp - 4k ap - 26/27% crit and 400 resilience with my group
buffs. Usually my main have less ap/crit and more stamina.
Arena...well
in solo mode is complicated for me. maybe in 4 with other player/healer
will be better, no great ambitions...but it s imposible do worst then
me actually:P
I can manage 1/2/3 killings, but in 5v5 often i
play vs groups with 2 healers (often priests/paladins/druids) = no
chance.
This is my current spec choice, so i can do my heroics with same spec/glyphs ( i play it with 1 prot and 4 retri).

Fear spell classes(mainly locks /priests) and mages are the most dangerous "enemies" for me.

All mixed strong Holy/retri. Dps is really low compared to retri. But
i couldnt do accurate testing with same level gear then retribution.
So, my opinion is that maybe can work, but ofc forget almost instant
killings. Tested in arena too. Really complicated to have a clear idea
there, i got just 15/16k hp and 300 res with holy gear. I failed ... a real fast fail.

All
full Prot+retri talents : really interesting spec. My gear is decent
only on my tank pala. Really solid vs melee classes.And a great
survivability. I invested a lot in stamina with this spec. 27-30k hp
buffed for all. using some prot gear.
And with talents it becomes a
real Hammer of Justice festival. You have more spells/ability then
retribution, so more creative rotation.
dps is lower, but
consistent . SoR does pretty high dmg and eventually with glyph mana
cost is really low. Av Shield is a great start cast. Mana is an issue
in long fights ...and ofc impossible heal much .
And, at the end,
consider Blizzard going *nerfing*/changing several prot tree talents in
new patch. with bad effects for a pvp spec.
Played many bgs and arena with this spec. really funny.

I havent tested mixed spec group (holy+retri etc) simply because for me in pvp can work only in static/defensive situations. really complex. And atm, havent decent healing gear.

i burnt maybe 20k gold for these tests. but we working for science:P and knowledge

Final
words with some idea(s) : i would like learn to use divine
sacrifice+divine shield (in combo -40% dmg) on each char in rotation. atm cant
get working decently but can be an interesting dmg reduction method for
gain better Survivability, for me a great issue especially in arena.
All melee, i have great issues with targetting. really confused by changing targets etc. currently playing 5 druids in melee mode. target visibility is a lot better. maybe can paladins play in cat positions in future patches please?