Will upgrading to a 32 GB RAM Macbook Pro significantly speed up Adobe Illustrator?
Judging from the stats you provide, it seems like it may be a CPU-related issue. Try to investigate this further to understand better if the issue is RAM or CPU related. 

To determine CPU use, go to Activity Monitor and press cmd+2 to get the CPU usage graph. See if one CPU is peaking during these operations or if it is multiple CPUs. 

According to Geekbench, your MacBook Pro actually has pretty decent performance, however the newer model that you have in mind, would be 15% faster for single-core operations and 46% for multi-core operations, respectively. Moving to an iMac could offer a performance boost of up to 34% and more than 100% for multi-core, respectively.
Since you are considering to move to a stationary iMac, you may also want to consider the Mac mini. It currently offers the most "bang for the buck" as it is cheaper than most iMacs. You can even save more money by installing additional RAM yourself (which requires some tinkering) and use external drives via USB-C.

To diagnose RAM usage, have a look at Memory Pressure, which is indicated at the bottom of the Activity Monitor app, when Memory is selected. If it is yellow or red, adding more RAM may actually help. 



In addition to that you may want to have a look at Adobe's guidelines on Illustrator performance. Finally, there seems to be / have been (?) an issue with performance on your model and users reported that disabling the dictation feature in System Preferences > Keyboard > Dictation helped.