I don't know what they mean by "true cfm". That's not scientific jargon, that's sales and marketing. I know that there are different testing methods to come up with a rated cfm, so you can't necessarily assume one manufacturer's rating is the same as another. But 1,800 cfm seems very low. It's like comparing different dyno numbers or flow bench ratings. What one dyno says and what another one says may not mean the exact same thing. But, they should be in the same ballpark. 1,800 cfm isn't in the same ballpark. That's half.
But yes, I wouldn't be too stressed out about a car that's holding and maintaining 210 at idle. It is a little high considering I don't think you're dealing with ambient temperatures that are all that hot yet, but with a new engine and a car that hasn't been on the road yet I wouldn't be buying new fans just yet. It's worth watching, but I think you have more tuning to do.
Yes, the grounds are very important. I mentioned this earlier I think too, if the fans aren't able to pull the amperage they need, you aren't going to get that rated cfm. And if you have poor grounds, you won't supply the needed voltage. It could also mess with the sending units, but I think you need to confirm your temperatures with an IR or other methods to make sure the discrepancy isn't the gauges. Could be the fans are operating fine and the temp gauge you're looking at is off.