I evaluate displays to determine if it is a product we are going to carry. Performance is one important attribute but there are others factors as well. I also get to calibrate them on our installs. You can't help not making subjective observations but we always do measurements and calibration for our evaluations.
As far as how measurements are taken there are many different ways. I'm not going to go into precise details (too long) but be assured 'ANAL' is a word I often hear associated with my name
To evaluate a PJ I take measurements from the PJs so I don't involve the screen or room. To calibrate a system I measure the screen as the room and screen now need to be accounted for as part of the system.
If you are a black level fanatic the Sharp 12K/20K, 3000, 3100, JVC RS-1(HD-1), Marantz S4/S1, 4001. The new Epson 1080UB impressed me at CEDIA for an LCD, it should be shipping soon. I'm curious to see a production model. I'm not terribly fond of the dynamic iris that many LCD and LCOS use.
I think the 710 had very good black levels for its day and price range but there have been significant advances since then. I don't doubt Jason's measurements or evaluation. He uses very similar equipment to myself. I wouldn't compare measurements of different reviewers though. Measurement techniques and set ups do differ. Also, there is a unit to unit variance with any PJ and bulbs vary quite a bit. If a reviewer is meticulous you can trust reviews from the same reviewer for comparison.
What the 710 has in its favor is a strong ANSI CR and great shadow detail. I'd personally take good shadow detail over the ultimate black level, YMMV. A PJ with very good black levels might be crushing some of the finer gradients and detail in the dark areas to achieve its black level. Where ultimate black level really shines is in scenes with star fields, space shots, credits and anywhere there isn't a lot of fine shadows but bright objects on a basically black backround. Ideally, we want both but this can't always be achieved at all price points.
The 710 is DC2+ and Joe Kane didn't want to use DC3 for other reasons. There were some tradeoffs with it but I forget the reasons he told us. It was at a class. I don't know Joe personally. But, there is a lot more to the chip than just the mirrors ie. processing capability of the chip, size of the mirrors, speed of the mirrors, pixel gap, timings, etc.
As far as the 7210 goes it isn't going to measure the best. It wasn't designed for that. Plus small improvements in black level make big improvements in CR that don't necessarily portray overall PQ. For instance if I were to measure .04 ftl black level with 20ftl white level I'd have a CR 500. If I could lower both .02ftl (.02/19.98) my CR almost doubles to 999 and I still have the same dynamic range. What if I went the other way (.06/20.02) my CR is now 334 with the same dynamic range. Small differences in black level make big differences in CR.
If I use HT Mag as a reference, in your example the 7210 CR is 1423 (.018/25.62) but with the Sharp 12KII the CR is 3623 (.002/7.263) and although the Sharp's black level is 9x better the Infocus brightness is 3+ times greater and the 7210 has a greater range. Now ANSI also plays a role, in HT MAG measurements the 7210 (419) has less ANSI CR than the 12KII (525) but the 7210 has greater ANSI than Sharp 3000 (396). But, the Sharp 3000 has a significantly higher on/off CR (6095) than either the 12KII or the 7210.
I have yet to see a projector that had the brightest whites along with the darkest blacks. What I have been rambling on about is contrast ratios don't tell the whole story. There are many other factors ie gamma, color saturation, color decoding, color accuracy, processing, etc. Don't use CR alone for comparing displays.
Manufacturers must pick their tradeoffs when designing a PJ. Infocus designs there PJs to be bright knowing it will hurt them in marketing. In non dedicated rooms (ie bat caves) the room and screen are going to to have a greater affect on CR and perceived CR. The extra lumens might be a better option for some of these situations depending on lighting, screen type and size.
Hope this helps