20% in my opinion are barely noticeable and not worth spending money.
I would by an refurbished 3 year old workstation with warranty, sell whatever professional CAD card is in it and sell your old system and use the money for an faster GPU.
Your system isn't that bad, actually. My backup gaming system is slower, but in a pinch, it still works.
Did you upgrade to SSD? This was what I perceived as the most significant and cheap speed upgrade.