Well first off matt, I hope you're getting paid for that extra hassle.. DAMN that's a ton of work!
The first report you want to look at is the one that GM uses for your official times.
Go into globalconnect, to GM Certified Internet Dealer under the Sales workbench (get your PSC to add it if you don't have it) then click the reports tab at the top and look at the Monthly Lead Performance report. This is where GM gives you the official numbers that you are held accountable for in a nice pretty PDF. Unfortunately for me it's friggin january and earlier haha. Hope you can at least see february.
Next step is to find GM Lead Management also under the Sales workbench. This will have much more detailed reporting. The first report is SFE Digital Performance. This will again, give you an overview but it will give you values based on each quarter.
The next one to look at is SFE Leads Not Responded to. This one is huge, any lead not responded to will skew everything else. So with this report you can figure out exactly which customers weren't responded to according to GM and you can usually figure out where the problems are.
If you need more detail on this, the Leads Not Responded to Detail report is harder to use, but definitely gives more detail.
Finally, look at the Dealership Leads by Source and see if there are any problem areas that need to be addressed.
Also, call up GM CID support at 888.462.8976 x4 ... make them help you find the right reports!