Far more capable people that could add to this - but will give it a try!
There will be a day and night difference between a 2011 cMBP and a 2014 rMBP in terms of performance. Upgrading/maxing your RAM on a 2011 machine, won't make much of a difference in terms of making it "faster" - whereas replacing a HDD with a SSD will make a massive difference...
BUT - there are all sorts of caveats to this.
Like what might be making your current MBP slow in the the 1st place?
And what exactly do you mean by "running slow" and over-heating?
In most cases, something could be sorted out with the OS, that would ease most of your problems. Upgrading that 750GB HDD to a 256GB (or larger) SSD will make it feel like a new machine, and some extra RAM (depending on what you do) might also help - but it will really depend on your use!
Also - keep in mind you would be dropping your storage space dramatically, but you would be winning on a fantastic screen, and a significantly "faster" machine...
In short - you'll probably need to be a bit more specific on what your needs are, and what's currently happening to your MBP, in order to shed light on what you could do... That's before getting to the whole question of 'what you need vs what you want'...
Sometimes it takes ages for an app to launch for example. Most of the time I hear the fan getting very loud and at that times it starts overheating, so much so that it is hard to keep it on the top of my lap. I have Yosemite Beta installed now but I had these issues with Mavericks too. Starting applications like iPhoto and Chrome is a nightmare mostly. Recently I dumped Chrome and started using Safari. It is much better now.
MacBook Pro is my secondary Mac so I don't do any work related stuff with it at all. The things I mostly do are reading/sending emails, surfing the web, listening to music, using Beamer to stream movies to Apple TV etc.
Since I'm not happy with having two computers any more, I plan to sell my iMac and purchase a Thunderbolt Display to hook up the MacBook while working.
I do web work and on my work computer most of the time I have these apps running: Coda, Safari, Chrome, Firefox, Skype, Textedit, Dropbox, Fantastical, Reminders, Readkit, Twitter, iTunes, and sometimes Suitcase Fusion 5 and Photoshop.
So what I'm mostly wondering is, would rMBP with i5 be better than my old MBP with i7? Does it all boil down to having SSD or not? What about 13" MacBook Air early 2014? How would its performance be compared to my existing MBP and rMBP considering the work I will do with it?