Can I upgrade my MacBook Pro (mid 2012) Ram to 16GB?

If it is a Retina model no. If it is not a Retina model yes.

You could also increase performance by changing out the HDD for a SSD.

An A1278 is the non-Retina model so, with RAM from a trusted Mac RAM vendor, you can get to 16GB. RAM ffromr the local "sell-it-cheap" store or web site may not work.

I buy all my RAM from Other World Computing. My only other trusted site for reliable Mac RAM is Crucial.com

• Adding RAM will not make you Mac hugely faster.
• Changing the mechanical hard drive for an SSD will make it neck-snappingly faster.

I upgraded my A1278 to 8GB as soon as its warranty ran out and noticed no big speed difference. A year ago I pulled the slow 500 GB SATA II (3GBps) mechanical hard drive and replaced it with a 500GB SATA III (6GBps) SSD from Other World Computing. I feel like I have a new computer. Slow boot times and load times are gone! PS Elements used to take 15- 20 seconds to load. Now it is ready to use in under four seconds.

The "slow" in this model is because it was an entry-level model and Apple installed a less expensive 3GBps hard drive even though the logic board has a 6GBps drive interface:

{image is from a 2011 iMac with the same issue}

Now my negotiated link speed for the SSD is a very nice 6GBps:

In that model, replace the hard drive cable at the same time you replace the drive. The cable is prone to vibration wear over time--3 to 5 years and it can cause symptoms that mimic a failing hard drive.

This model is one of the easiest to work on that Apple has made since they went to metal cases.