Im sure if measured with calibrated mic setup we’d be able to find a difference but in terms of an audible difference to the vast majority of the public a couple of mm shouldnt make much difference, as long as all the edges meet for an air tight seal, thats what matter more
For a woofer with port tuning anything below around 240/250hz the wavelengths are much bigger than the overall speaker so a mm or 2 wont matter, 240hz has a wavelength 1.43m and they get longer as frequencies get lower
If you were building a more complex multi way speaker with a mid range driver (for example 600hz-2500hz) that needed its own smaller enclosure within the main cabinet or for some reason a tweeter that isnt already enclosed (if they exist) it would probably matter as you start dealing with frequencies having wavelengths shorter than the dimentions of their drivers respective chamber(s)
For more context, even if you were batch building speakers to sell the dimmentions still dont have to be exactly precise after conversion they would just have to be consistent unit to unit
EDIT- Also it’s occured to me that most people dont even think abbout the internal volume they lose when they add bracing and that its usually much more volume change than conversion accuracy losses in this instance
