When I did my preseason team projections, I included a spreadsheet that has, among other things, defensive projection totals by team. The spreadsheet can be found at the bottom of this page. I didn’t have any team projected better than +38, or worse than -24. I took some heat for that, most notably from TangoTiger, who thought the spread of defensive runs should be doubled. I didn’t see how that could have made sense then, and I don’t see it now.
Projecting a single player or a single team is always going to have plenty of variance, but for groups of teams projections should be pretty close, and if they aren’t then something needs to be fixed. So I looked at the top 5 projected defensive teams, Boston, Seattle, St Louis, Oakland, and Cincinnati. Then at the bottom 5, San Francisco, White Sox, Minnesota, Pittsburgh, and Toronto. At this point in the season, the top teams should have a prorated defensive total of +12.4, with the bottom teams at -5.9. So far Totalzone (found on baseball-reference) has those top 5 teams at +10.8, and the bottom ones at -3.0. If anything, not enough regression, but this might just tell you Totalzone can predict Totalzone, which is not the same as projecting team defense.
Looking at UZR from Fangraphs, the top teams are +9.2 and the bottom teams are -3.0. Looking at Defensive Runs Saved by Chris Dial (Baseballthinkfactory), my top teams are +7.6 and the bottom teams are actually not that bad, +1.2.
The only thing I can conclude for this is that artificially boosting the spread of runs in Totalzone (just calling every +5 fielder a +10, etc.) would be an unmitigated disaster. I’d lose a lot of accuracy in not only predicting future TZ for teams, but predicting what other, independent systems will show for teams (and maybe even actual defense). Also, the San Francisco Giants are worthy of some looking into. They projected as among the worst defensive teams, but have actually been, by all three systems here, among the best. TZ whiffed on the Giants. Not sure what projections others had for them.