There is already a supergroup of rad/rad defenders on Union called Rad Rangers, they have a thread in the Union forum and are coalitioned with the Precision Rangers, SNHS and many more!
G-Force
CAN I JOIN PLEASE!!!!!!!!!!!
Tell me where and when to be on!!!!!!!
PLEEEEEEEEEEEASSSSSSSSSEEEEEEEEEE!!!!!!
Rad teams are perfect and there isn't anything wrong with everything you listed... except you forget there's more ! 
8 stacked AM :
+200% damage
+200% recharge (3x better than hasten)
+200% end regen (4x better than stamina 3-slotted)
With 8x assault, it means everyone will be at the damage cap as soon as lvl 8, (as you should take EF at lvl 6) and with 8 stacked EFs the -res makes it equal to 1000% damage.
Maneuvers got neutered in I5 though, it offers like 4 or 5% base def for defenders.
However... you've got a huge accuracy and defense debuff in Radiation Infection - it's base is -30% for both effects. With only 3 people, everything up to purple will be at 5% to-hit. With 8 people using it... let's say you won't need accuracy enhancements, even while fighting +6.
In my experience, every minion (up to +6) and lieut (up to +4) die at the first "AoE wave", leaving 2 boss looking around very lonely. 
Oh, and about the "8 healer" part? In my rad/ super-team, no one took the heal. Ever. I'm not kidding, stacked debuffs are that good. 
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CAN I JOIN PLEASE!!!!!!!!!!!
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Look for the Rad Ranger thread in the Union-Heroes forum and introduce yourself. I'm sure it can be easily arrenged.
G-Force
The more the merrier, we're having a rad rangers day on saturday which should be a lot of fun...
Off on holiday 
Im Rad / Electric, but that shouldnt matter IMO as electric also have 2 good AoE blasts, and personally i prefer END drain to acc debuff (which arent really needed if you have Rad primary).
Still welcome?
What is really needed tho is the determination amongst defenders to get assault tactics. I have been on CoV with just 3 or 4 MM all with assault and they absolutely RIP through minions without pause.
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... and with 8 stacked EFs the -res makes it equal to 1000% damage...
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Resistance debuff has a cap at around 250%, so 300% +250% =550%. You cant get it higher.
A Paragon Defender
Defender damage cap is 400%, not 300% ; it's possible to be capped as soon as lvl 8 (8 assault, 8 AM).
A res debuff doesn't add to your base damage, but to total damage, so you have to multiply instead of adding. (Btw, the math doesn't work the way you said, for example a -30% res debuff is a +22,5% boost to your total damage)
Obviously I was wrong when I said 1000% (had the old -40% res EF in mind), but it's still around 700+% damage.
Just started a Rad Defender - now lvl 10, im a fairly experienced player i must add, I just fancied a different type of Alt.
Loving Rad i must say. But listen to my fantasy:
8 man team of Rad / Whatever (except anything that knockbacks)
All on team have Assault + Manuevers
All on team have Lingering Radiation + Enervating Field
All have fitness pool / stanima
(OK Tight I know, but bear me out)
I'm not really a min/maxer or huge into the math but...
From Tactics Pool alone: +80% defence, +100% damage. So, team are already all tanks, and do more damage than 8 blasters without tactics.
Team runs to mob. Bam Ennervating Field x8, Bam Lingering Duration x8 all synchronised. Alpha Strike? pfff can you imagine the effect when running 80% defence and suddently 8 stacked dam red effects on team.
Defenders all run into melee (to keep group clumped). Fire of 8 AoE blasts, all at +100% from assault, against mobs with 8 stacked def debuffs and 8 stacked dam res debuffs. I can only imagine anything that isnt +4 or +5 minions will drop in two rounds of this.
Then drop the anchor (a lt or boss)
Mop up stragglers
Move on
Anybody see a problem with this? I racked my brains. Teleporters, maybe, but not many and the odd one can be endured. Toggle droppers? Maybe, but remember 80% defence, they can't drop more than one or two? And recall 8 healers in groups to account for errors.
IMHO this team would absolutely tear through anything on invincible, maybe even strike forces.
If anybody is interested in this, I will set up a SG of rad defenders on Union. If enough join, the only thing stopping team would be lvl cap.