Well, we can factor in criticals if you want.
Plasma Grenade boost per 1% of crit chance = 0.65%
Avenger boost per 1% of crit chance = 0.5%
Boost on Avenger with 2 BRD per 1% = 0.6
Now its very easy for a Plasma nader to get near +25% crit chance, they have very few requirements and no specialised perks. Lets say each has 3*More crits (Finesse is a stupid idea for burst, so ignore this), BG has 2*BRD, 3 More Crits, BRoF and ... Toughness? and Grenade Thrower has Silent Running, Thief, Heave Ho!, BRoF and 3 More Crits.
My Sneak Thrower would be: 3/6/4/1/10/10/7 - Small Frame/Fast Shot - 95% accuracy at max range, enough sneak to be 15 hexes invisible vs Pe 10, crit chance of 22% (or +14.3% damage).
BG Char we have 7/6/6/1/6/8/7 - Small Frame/Fast Shot to compare fairly, though Small Frame is less convienient for BG - Let's say this will do, we don't want to waste the perk on weapon handling because more crits gives +5% where +3 Luck is only +3%. Crit chance also +22% (or +13.2%).
Since you say Toughness (1) is the norm:
Plasma Grenades Average Damage = 65, Metal Armour II is 4/25%. 61*0.75*1.143 = 52.3 HP
Avenger = 11.5, Metal Armour II is 6/40% = ((11.5*0.6666)-2)*0.92)*13*1.132 = 76.7 HP
3 Avenger Shots is then about 230 HP and 5 Plasma Grenades will be about 262 HP, even 4 Grenades are 209 HP.
Having tested it it seems the animation speed pretty much caps it at 5 Grenades per 3 Minigun bursts - This means a Plasma nader is like a Minigunner on Jet only they do more damamge, costs them nothing and the animation speeds are near equal. (I think its a pretty safe bet Jet won't be allowing you to gain 6 AP in future either, less effect and a longer period seems to be the best way forward.)
Advantages the Ambush Nader has are very high DPS, splash damage, the ability to choose where combat take place and very little risk/set-up cost (say $1000 vs $12200). Sure you're going to want Armoured people somewhere, but 3 or 4 Grenaders are a massive force multiplier (and still would be even if the only thing to change was fast shot not working on them).