The yeast performance is better with increased calcium, however I can confirm that the flavor of a Czech Pils will not be improved with more calcium. With proper practices, the lack of 'sufficient' calcium in wort can be overcome. Many of the mega brewers use water that has less calcium than desired in typical brewing. So it can be done.
If you look at the list of benefits of calcium to brewing you usually find things like:
Reduction of mash pH
Protection of enzymes against heat
Clearer runoff
Better protein coagulation
Better yeast flocculation
Precipitation of calcium oxalate
Less silicate/polyphenol extraction
Less production of color
Beers that use low calcium are typically lagers that are acidified with sauergut or sauermalz. Most of the rest of the items have to do, ultimately, with beer clarity and as these beers are traditionally lagered for a long time those things that might not precipitate as well in the short term because of reduced calcium they eventually do settle out because of the long lagering times. One does not particularly want good flocculation with these beers because one wants to carry yeast in suspension over to the lagering vessel. Having pretty hefty yeast counts during lagering is required for effective removal of diacetyl and acetaldehyde. These beers tend to be darker in no small part because of decoction mashing but enhanced color from less calcium is something the brewer has to live with. No 3 and 4 SRM beers are produced (AFAIK) using this process. Protection of enzymes against heat is achieved in part because most of the enzymes are removed from the decoctions before they are heated.
I was reminded by my friends who were in Pilsen this week, that PU ferments in open fermenters.
Do they? I haven't been there in a long time but when we were ushered into the fermentation hall - large underground chamber with cold brine pipes suspended from the ceiling and hundreds of slightly inclined traditional wooden fermenters fixed to section of rail, those fermenters were empty. The beer is now fermented in more modern equipment. I assumed it is uni tanks but I don't know. We weren't shown the modern cellars.
There would always be a low level of oxygen in the wort in fermentations at PU.
A low level is desired except at pitching. This is insured, in an open fermenter, if the fermentation initiates promptly as the surface of the fermenting beer is quickly blanketed with CO2 which, as it is heavier than air, tends to stay put unless there are air currents to disturb them. Cellars for open fermentation are not designed to have strong air currents.
Contrast that with the near zero level our typical ferments create in carboys or cylindroconicals.
A carboy or unitank is full of air when it is filled. The same mechanism protects the beer one the CO2 starts to outgass. Of course once the air is fully displaced it is much harder for any O2 to reach the beer by diffusion in a carboy or unitank.
I need to find out if micro-oxygenation has an effect on calcium needs by yeast. Something to add to my 'to-do' list.
Yeast needs calcium beyond its effect on flocculation as a cofactor for some enzymes. There is plenty in the malt itself for that. The oxygen is added for sterol formation. Whether the enzymes in that pathway require calcium cofactor or not I don't know.
It is certainly possible to brew good beer with very low levels of calcium. I did it for years and still do though I now use a bit more because I want calcium oxalate precipitation to occur in the lagering vessel, not my plumbing. I've been through that once and once is enough.