EF-M lenses do have copy variation. Bryan’s (TDP) first copy of the M18-150 was terrible. I sent him images from mine, which is optically very good, he ordered another copy which was much better and that’s the one you see in his ISO 12233 shots.
As a rule, when you buy a lens test it! Checking for decentering isn’t difficult, and it’s a common cause of poor IQ that results from rough handling during shipment. There is also variation out of the factory, less with L lenses, more with consumer lenses, and even more with inexpensive lenses like Samyang/Rokinon.
Precisely. I have owned multiple copies of the same model lenses, usually in part because the first copy was poor. I have also used multiple copies of same model lenses (e.g. borrowed from friends). I have seen first hand on several occasions that sample variety does exist, sometimes significantly. Usually less so with more expensive lenses, but even that is not an exact science / formula.
Thankfully my current lenses that I rely on for critical sharpness, are good copies, and do not suffer from de-centering. My Canon EF-M 18-150mm is a sharp copy, as is my Canon EF-M 15-45mm, but I have seen poor copies in tests, and from other users. My EF and EF-S lenses likewise are good. I previously owned a 2nd hand copy of a 10-18mm which was significantly soft at the 10mm, but relatively ok at 18mm. My current copy is very sharp at 10mm, and suitable sharp at 18mm (at least just as sharp as the previous copy). As I use this lens mostly at 10mm, it was a no brainer to keep this one.
My current most used L glass (Canon 70-300mm L and the 100mm f/2.8 macro IS) are both very consistent and sharp across the frame. Very happy with them, but I have used a 70-300mm L which was notably less sharp at 300mm than mine is.
Len Rental's tests (thanks Roger) of multiple samples of the same model lenses is really insightful. The most recent Canon lenses (including the 70-200mm F/4 IS II and the 70-200mm f/2.8 III are particularly good examples of close tolerances and good sample to sample consistency for image quality- i.e. sharpness, etc)
But back to the point - of the potential new EF-M lenses, yes I would really like a EF-M 15-150mm lens, especially if it can be maintained in approximately the same size factor as the existing 18-150mm - though I realise that's pushing the laws of physics quite a bit! I love the 15mm on the wide end, compared to 18mm. That's one reason why I use my 15-85mm on my 80D / 7D, etc.
No need for 'fast glass' for all purpose / kit zooms, for the way I use EF-M lenses, with my M5. I prefer using primes for really fast glass, e.g. f2 and faster, when I need that.