I haven't found a really good metric through which to gauge balance in Shadowrun encounters, and while I'm still testing things out I'm nearly-totally (almost) convinced that for Shadowrun there really shouldn't be such a thing.
I use Professional Rating as a guide for how experienced I should play the NPCs (which includes a rough estimation of skill and quality of gear though not always) as well as group edge (players hate it when they hear you make a roll behind the screen and then make a secondary roll right after counting hits; "oh crap, the GM's using edge!" hehe)
I plotted the BP (skills & attributes, not including equipment costs) vs Professional Rating for the Grunts & Lieutenants out of SR4A (only) and got these two handy (ha!, sarcasm there) equations:
Y = BP, X = Professional Rating
For Lieutenants: Y = 6.357X
2 + 34.857X + 180.21
For Grunts: Y = 3.3056X
3 - 18.024X
2 + 65.861X + 103.48
And the Lieutenant Prof Ratings of 2 & 3 didn't fit the curves "nicely." Rounding to the nearest 25th BP, they came out to:
Prof Rating | Grunt BP | Lieutenant BP |
0 | 100 | 175 |
1 | 150 | 225 |
2 | 200 | 325* |
3 | 225 | 300* |
4 | 275 | 400 |
5 | 400 | 525 |
6 | 575 | 625 |
* The Lone Star Lieutenant came out to 307 while the Corp Security Lieutenant came out to 325, even though the LS Lieutenant has a higher Professional Rating.
Surprise is deadly, cover is key, and information is life.
For example; the SK kill outside the chopper. With a 4+ Professional rating the SK leader would have probably called in to the (now dead) pilot, "we're approaching the tarmac, warm the bird up." If the PCs thought of monitoring the communications they have to Con the leader. If they didn't think to monitor the communications, well, now the whole SK team knows their pilot isn't talking and something's up and they sure aren't going to just waltz up to the helicopter like a kid to candy store

How different would the fight have been if the characters, watching stealthily from inside the chopper, saw the door to the helipad/roof swing open and only a small lone thermal smoke grenade jump out from the darkness beyond.
So basically, the higher the professional rating the more those NPCs are communicating with each other in ways that help to ensure their survival.
There's a great editorial article by Roger E. Moore in an mid 80's Dragon Magazine where he tells the story of the hardest dungeon he ever went on (1st ed. D&D). In short form; The characters were around 10th-12th level and it was an old-school dungeon where the first level has level 1 monsters, 2nd level - level 2 monsters, and so on. The PCs all want to make their way to the 8th - 10th levels because that's where the good treasure is. But the DM set it up that there was a small tribe of kobolds living on level 1...it was their home and the kobolds had dug tunnels, made secret doors and simple traps. Within a few rounds the DM had the 10th level characters running, screaming, from the kobolds. The little bastards would shoot and run, wait for the PCs to pass over hiding holes only to pop up and crossbow bolt them in the back all Red Dawn style, and so on. By the time the party got to the lower level the mages and clerics were out of spells and everyone dangerously low on hit points. The party rested, continued on, maked a great haul from the lower levels and then realized with horror, that they were going to have to go through the kobolds to get out. These are low-intelligence NPCs who just know their home and have adapted their environment to suit their needs...very deadly for PCs.
Even with a professional rating of 6 and the greatest gear...that NPC is going to be in trouble if he/she is successfully ambushed by the PCs. The more I GM Shadowrun the more scared I get at ambushing the PCs with deadly force. Surprised characters cannot dodge or react to that which has surprised them. So what was a Pistols + Agility vs. Reaction (or Reaction + Dodge) is now just a Pistols + Agility roll. Which means some punk with a 5 DV, -1 AP gun with laser sight and having taken aim, is rolling around 6 dice (2 for skill, 2 for Agility) and a really mean GM could decide to use the 2 dice from his Professional Rating of 2 as edge to roll a total of 8 dice with exploding 6's. Unless this punk rolls really well, on average (when not using edge) he'll have the PC resisting 7 DV at -1 AP. Now imagine 8 to 12 of these punks all waiting in their secret hide-y holes in their favorite alleyway as one of their members leads the PCs in a chase through this kill zone. Sure, the PCs might roll really well on the Surprise, but all the punks get a +6 bonus to their rolls for lying in wait and none of them will be surprised by the PCs. Sure it's only one IP worth, but it's a deadly IP.
What Medicineman says is also key and a good metric for using spontaneous/improvised NPCs. Their Professional Rating equals their Attribute and Skill ratings (then adjust for metatype and gear modifiers). And 2 to 3 times their PR (minimum 2) equals the number of active skills they have. All as a rough estimate for doing things on the fly. 0-1 PR probably no augmentation or magic. 2-3 PR basic augmentation and/or very limited magic. 4 PR alphaware and/or some magic. 5 PR betaware and/or good magic. 6 PR beta and maybe deltaware and/or superior magic.
And on a surprise attack, if the ambushers are able to deal 6 boxes of damage to a PC...that drops them by 2 dice to all pools until healed. So if it started out with nearly equal Attack Pool vs PC Soak Pools, the NPCs are now at an equivalent +2 over the wounded PC(s) which just made it far more challenging for the party.
A small insect spirit may be relatively weak, but if it manifests in the van with the rigger while the teams is elsewhere (good times), it can be very challenging.
Love it!

and also agree with Mercer.