How useful will Transcendent Grid be in Chicago, which hardly has any grid at all?
I checked the Missions FAQ but didn't notice any information on grids. With that said, I would think it would be pretty useful in a place like Chicago. I would assume that most of the CZ would be covered by the Public Grid or Global Grids. If most people are operating on the Public Grid, you would have a 2 dice advantage over them when using Transcendental Grid since you don't suffer the -2 penalty for operating on the Public Grid with TG up.
Honestly, I'm not sold on Diffusion of Firewall either, mainly since it is resisted with the very attribute you're trying to reduce, is sustained, and has typically ridiculous CF FV. Someone elsewhere made a compelling argument that Infusion of Firewall is not too shabby with a high-class commlink. Yeah, the FV sucks and you'll probably have to sustain it on your own most of the time, but Firewall 10 is nothing to sneeze at (particularly if the team's gear is slaved to it). Register a few sprites and have them sustain the CF when combat breaks out.
Edit: You
could ditch Cybercombat, but it means you're giving up a lot of your combat utility. You can get by without Brute Force and Crash Program easily (just Hack on the Fly and don't worry about the programs), but subbing Resonance Spike for Data Spike is not a good choice at all due to how Resonance Spike works (DV = net hits of Software+Resonance vs. Willpower+Firewall limited by level) compared to Data Spike (DV = Attack + net hits from Cybercombat + Logic vs. Intuition + Firewall resisted by Device Rating + Firewall). Against someone with average stats and gear, you're doing 2 damage with Resonance Spike and 5 with Data Spike. Granted, you risk taking damage if you fail and are going to raise your OS with Data Spike, but you have to resist fading every time you use Resonance Spike.
ETA: The other answer to the Data Spike/Resonance Spike problem is to just say screw it, get rid of Cybercombat, get rid of Resonance Spike, and make sure you have a Fault Sprite or two available at all times to do your fighting for you. You can jack with enemies while they fight using Pulse Storm.