So... change your stress test to 5e, same basic group. Does the outcome change materially? Dude with Grenades still kills everything doesn't he? Or at least does so much damage that clean up is a foregone conclusion. So why wasn't that happening at your 5e table?
Well speaking strictly for myself, switching our stress test to 5e would have been a world of difference. Primarily to your point, the elite Aztec PR8 NPCs I used would have all been in light hardened milspec, so would have been flat immune to grenades without chunky salsa help. Revenant would have also been ludicrously more effective given the rules for MysAds in 5e vs. 6e, and Sovereign would have had several more spells including combat ones.
All that aside in a more general sense though, it would have changed even for average runners average opposition. Same base damage for the grenade (with slightly different rules for diminishing damage based on distance from ground zero), vastly different average soak pools. 6e, average HTR has soak total around 7. 5e, average HTR has soak total around 24 (body 5, full body armor 15, helmet 2, ware 2). Average difference of 6 is a big deal for grenades doing the same damage.
Well, if you put the 6e goons in Hardened armor they'd have lasted longer. Milspec/Hardened armor just isn't printed for 6e yet. : ) I would argue very strongly that using grenades, especially indoors, in 5e would have turned anything without Hardened Armor (Milspec, or ItNW or whatever) would be pounded to goo. Might have taken a second or occasional third grenade if they're real spread out, but they'd all still be down in a single turn. This is generally what happens with a table of well built 'runners. "Reasonable" opposition tends to go away almost immediately. Unless the GM throws out something stupid, Alter Boy, dozen force 10+ spirits, couple of panzers, Dragons and Juggernauts...
Yes Chunky Salsa rules are a pain. Yes I'm glad they're gone. Still made 5e Grenades every bit as mechanically effective as 6e Grenades, IMO anyway.
My last bit, mostly for Missions play as I think stuff like this can be worked out at a table. "Jeff, knock off the grenade spam please, we get it." I feel the social aspect of the game is strong enough that unpopular tactics (Grenade spam, monowhip weapon foci, 5e Rigger SWARM rules, Pornomancers, possession traditions, high grade initiate mages, ect.) will restrain themselves most of the time and not disrupt the table for the other players/GM.
That's been my experience as a Missions GM and player. Players with mechanically strong characters get a feel for the GM/table, and play for fun. We have a mix of high karma and no karma characters at any given table, and 99% of the time everyone has a good time. Problem players will always be problem players, GMs will have to deal with them.
Missions can identify the unbalancing / unfun mechanics and put some rules around them in the Mission FAQ. Did it for 5e. I'm sure it'll be done again for 6e. For grenades in 6e Heat may work, raising the availability may be the way to go, or just putting chemsniffers everywhere. Just something there to enforce the social contract for the borderline cases. "If you do X, Y bad thing will happen so lets find another option..."
I'm all for mechanical changes to the way grenades work in 6e to bring them in line with other options. But Pepper Punch Grenades, AoE spells, Spirits, Multiple Gun Bunnies using Anticipate.... I think even if you took explosive grenades out, 6 well built runners will chew up a lot of NPCs real fast.
In general, nerfing "The best option" quickly becomes a game of whack-a-mole as there is always a best option. I prefer to have several "Best options".