Depending on what you do, the difference in performance could range from insignificant to tremendous IMO. What do you use the system for?
For example, the SSD in the new 2017 model is about 10 times the speed of the SSD in your current system - it's a difference of fast versus stupid fast, and there would be a very noticeable difference if you work frequently with large files, use virtual machines, or are exhausting the 8 GB of RAM you currently have. The CPU in the new system is a reasonably significant improvement, and GPU is a huge improvement, but neither of these improvements would be very noticeable with something such as Office or iWork, where as they would be drastically apparent with photo editing, video editing, streaming UHD content, running virtual machine, etc.
Other things that may or may not be of value to you include: slightly lighter weight when carrying, likely twice your current battery runtimes, much faster battery charging times, improved speakers, the capability of leveraging extremely fast external SSDs using USB 3.1 gen 2, enhanced productivity with the Force Touch trackpad, improved speakers, and a slightly improved screen that is brighter when working in very well lit areas with better anti-glare tech.
On the other hand, there is also subjectivity regarding the keyboard-some love it, some despise it, and some just consider it to be a keyboard. Additionally, you may or may not want to transfer to all USB-C (the benefits outweigh the frustrations for me, personally - but for someone who must frequently use USB-A, Thunderbolt 2, and/or SD cards, this can certainly be a source of more frustration than it is benefit.)