I read the review. It is really making me reconsider the $3k Epson 6500. The comment that struck home the most was about rooms that a not 100% treated lose a lot of black-level detail inherently. Since painting the ceiling black is not an option, the article suggests I will lose most of the difference between the 6500 and HD20.
FWIW, my brother has a plasma, and the vast improvement to black detail compared to his RPTV is super easy to discern, and the room it's in has a skylight.
My ceiling is untreated, yet the black detail is phenomenal (to me). I know it could be improved upon, and has quite a ways to go compared to say a Kuro Elite, but, well . . . I guess it's just really hard to say, and YMMV.
Would a gray screen improve the black levels for this projector?
You should understand, and you probably already do, the difference between absolute blacks and black detail. Obviously, if you calibrated for the best possible absolute black, you might lose a lot of shadow detail, so to speak.
I quite believe, and I bet many would agree (but who knows) that it's the detail between the varying blacks that are much more important than the absolute darkest it can get.
All that said, and this come from someone on his first PJ, most of what I have read leads me to believe that getting a gray screen is going about it backwards.
IOW, buy the PJ with better black detail. If you can. If you can't, just get the PJ that fits affordably into the budget, otherwise.