Can I insert 16Gb RAM into MacBook Pro early 2011?

"Yes.

If this answer was helpful please remember to return an mark it Accepted."

"Will it work?

Yes.

Is it a total waste if you still have a spinning hard drive?

ABSOLUTELY!

Get an SSD, then RAM. I would rather have an SSD and 2 GB of RAM than a hard drive and 20 GB of RAM. As long as a computer is still using a hard drive, it is broken, as far as I am concerned, and miserably unusable."

"I have a early 2011 macbook pro 15"" 2.0Ghz and I installed 2 8GB Corsair modules and it recognizes and works.

But I have not, at all, noticed any improvement in the performance :-(

I think If you upgrade to 8GB will do the trick..

In a few machines it just won't use all of it. I don't know, maybe it did and I haven't noticed."

Yes it's possible. Only be careful with the brands. Not all brands will work perfectly. I had some cheapo's working perfectly and had some not working at all.