Although it is technically okay to mix ram sizes (if the mobo and BIOS allows it), there IS a performance penalty when not using matched pairs of memory. That is true for just about any computer (Windows PC or Mac); , it has to with interleaving and other factors. In some cases, even if both sticks are same RAM size but different makers or configurations, that can also cause a performance hit, hence why most recommend using matched pairs of SAME manufacturer / model. By configuration i mean a 4 GB chip may have a 4 x 1GB config., where as another 4GB chip from another maker may have a 2 x 2GB config or 8 x 512MB (or whatever, ) Macs have historically much tighter tolerances (ie. less forgiving) when it comes to memory vs say a garage mix and match PC build; I strongly suggest using matched pairs by same maker, and stick to reputable brands. there are a lot of gray market chips out there with very questionable tolerances; if the price SEEMS TOO GOOD, THERE IS A REASON.

It amazes me why some are so hung up on wanting to upgrade internal storage or not having room for 2nd SATA/SSD. they are locked into ‘old school’ thinking. Complaining that the MM is not modular; it is, you have to step back and think outside of the case, literally and figuratively. The MM has thunderbolt 3 ports guys! The modularity is now EXTERNALIZED. Add storage externally at TB3 speeds, as MUCH as you want, WHEN you want. With TB3, other components can be externalized (GPU) without having to crack the case. By reallocating the internal space inside that would have been used by bulky SATA drives and extra storage, it allowed using better higher class CPU’s without increasing the form factor in order to help with cooling. On other sites were complaining the RAM was surrounded by the cage on purpose by Apple to discourage upgrading; no, it was too reduce EMI so user could benefit from faster RAM, and AGAIN, without having to increase form factor to provide more separation.