RAM could be an issue. Next time you're working in Logic, open up "Activity Monitor" from your Utilities folder and look at the Memory tab. Watch the Memory Pressure graph as you're working. If it stays in the green, you're good for RAM and more won't help. If it's yellow, some more RAM would help a bit. If it's red, more RAM would make a big difference.
The current rMBPs are only one CPU generation newer than yours which is good for a 5-10% boost. The next iteration of the rMBP should see another 5-10% bump on top of that. Not huge by any means.
If Logic is not crushing your RAM, it's probably a software issue slowing your computer down.
Excellent information on him watching his memory pressure. Although today's 15" rMBP's are two generations newer, with a significant update to both the iGPU and dGPU. Regardless of opinions on the AMD, I was in the same boat as the OP - albeit not for lack of storage. I have, still my 2012 15" 2.7/16/768GB machine and it truly does still fly! I do audio and video production to pay my mortgage as well and these machines have really made life enjoyable again, while working

and the original rMBPs are indeed still killer machines.
That said, I debated the OWC replacement as well and just before Christmas, I was at Best Buy. Their MacBook sales were phenomenal and 300 off the higher end builds 500GB+ machines. As well, a hundred dollar BB $100 gift card and up to $200 for a working but no longer used Mac. I brought my wife's 2007 white 13" core2duo (2.16?GHz, I believe) and was given $200 more, off the CTO 2.8/16/1TB. $3,199 that day on Apple.com. @ Best Buy - $2,699 @ BB (2599 considering $100 @ BB is like grocery money anyway these days

) - it was a bargain. I happened to have nearly a hundred bucks in BB myReward points as well, so I paid about the price of the $2,499 step up to dGPU/512GB.
The new model is also available on apple.com in refurbished for about $2700. They're a bargain as they've essentially been built twice.
The biggest change I've noticed is the hyper speed PCIe SSDs, 2,100Mb/s read and 1,600 write on the decent size video files, it's a difference that seems magic. TBolt 2's peripheral/break out boxes are also in excellent supply as are Thunderbolt 2 peripheral audio and video production hardware that destroys my FW800 transfers of a couple years back
I don't care about the Skylake machine. My '12 still puts a smile on my face. My 'mid '15' though, I have to wear a helmet!! Safety first and its
fast!