asymmetric ram

It is not very clear which version of MacBookPro you do have.

however, this page reports the maximum memory possible for every Mac model:

http://www.everymac.com/systems/by_capability/actual-maximum-ram-capacity-of-mac s.html

If your model is listed as 16-Gb capable, go for it (of course, wth 2 identical 8Gb modules).

I did on my late-2011 17-inch MBP (8.3), and the benefit was substantial....

It is always better to employ two identical memory sticks (same brand, same size, seme version), for avoiding timing problems and getting full dual-channel operation.

Your are just going into a can of worm installing a new 8GB stick alongside your old 2Gb stick...

In terms of RAM speed, going asymetrical will SLOW DOWN your machine, which is now working with superfast dual-channel memory access, whilst with the asymmetrical configuration this super speed will not be possible anymore.

So I suppose that you will be disappointed changing just one memory chip, the machine will be SLOWER than now...

More RAM means more speed only when you run out of memory, and the system is forced to read/write data from the mass storage. A situation which should be avoided as much as possible, so in this end maxing the RAM at 16 Gb AND DISABLING AT ALL the virtual memory on disk is the only reliable solution.

This is also quite important if you replace the HD with a fast SSD (which is better not to use for virtual memory, as this ages it prematurely).

My machine had a big speed increase, because I simultaneously installed the 16 GB RAM, and a fast SSD. And this is what I recommend to every Macbook pro owner. Buy 16 Gb of RAM (Crucial, possibly), and an M500 960Gb SSD. You will get a completely different beast, booting in less than 15s, launching heavy apps in 2-3 seconds, etc...

Ok, this upgrade is expensive, but I can ensure you that it is plenty worth the money.

Doing it just at half (expanding the RAM without the SSD, or vice versa, or, even worst, changing just one of the two memory chips) is as wearing two different shoes... You will never walk properly...

What is important that the specifications be what is prescribed for your MBP. A 13" 2012 MBP RAM specifications are: 204-pin PC3-12800 (1600 MHz) DDR3 SO-DIMM.

Though matched sets of RAM are preferred , there will some performance degradation in a mismatched situation that you describe (2 + 8 GB chips) as opposed to a matched pair of the same amount. However no harm will come to your MBP and overall performance WILL be increased in those situations where more RAM is needed.

IT WILL NOT SLOW DOWN YOUR MBP.

If your MBP has a need for additional RAM, go ahead and purchase and install the 8 GB RAM chip with a 2 GB RAM chip that yopu already have. When the bank account permits, purchase another 8 GB chip, and then you will have maximum RAM efficiency that your MBP can offer.

Ciao.