It is true that Velvia can look better at EI40, but a lot depends on the conditions under which it was exposed. It also looks fine at 50, just more contrasty (which is a problem with the Velvia reversal stock).
My shooting on Velvia does vary from EI40 (or less) to 50 (or more), depending on prevailing lighting. Rainforests, which form a dominant component of my work, are by nature very contrasty, even with a slight change of illumination, and my shooting is as far as possible organised for conditions where diffuse will ensure the best possible results, with carefully considered exposure. In these conditions Velvia is fine at 50. Recently I shot scenes in quite poor light at EI40 where an element of doubt niggled, then another at EI32 (EI40 was best, with better detail in marginal shadows). Why two? My Ilfochrome printer will have 2 identical trannies at different ratings to experiment with (any tranny will still require contrast masking).
It's best for you to experiment at 50 and 40 on various subjects, but especially the subject you specialise in, noting however that Velvia is not by design a bright (point) light film, though many artists do exploit this in a fun way where its gaudy colours clash and bang. Exposed carefully at any rating (EI64 is also common for very flat light) it delivers the results it is renowned for. Occasionally there are reports of "interesting results" with processing; don't get too carried away with options in processing i.e. push, pull over- under-development. Velvia is fine 'straight-through'. One thing to be cautious of is that longer Tv speeds can blow highlights at EI40: more detail in shadows often at the expense of highlights. At least this is much more graceful than D***!
Canon's EOS Evaluative meters (right back to the EOS 1N, which is my weapon of choice) can be considered extremely accurate, but partial and spot metering on Velvia in difficult light are also exceptionally useful.